miatoll_defconfig: Disable redundant Spectre variant 2 mitigations

Our big and Prime clusters are currently getting software mitigations
for Spectre variant 2 (CVE-2017-5715) applied through Trusted Firmware
despite the presence of Arm v8.5-A hardware mitigations. Disable the
software mitigations since they're redundant and are only hurting
performance.

Details and analysis:

The Kryo cores used in the aforementioned clusters are semi-custom
Cortex-A76 derivatives [1]. According to Arm, newer revisions of their
reference Cortex-A76 designs (r3p0 and newer) are immune to Spectre v2
thanks to hardware mitigations implemented as part of Arm v8.5-A [2].

While I was unable to locate a working Spectre v2 PoC for AArch64, Arm's
overview suggests that the v2 and v3(a) mitigations come together as part
of the single Arm v8.5-A update [3], so we can test for whether the cores
are susceptible to v2 by testing for their susceptibility to v3 and/or
v3a. This is helpful because there *is* a public and working Spectre v3a
PoC for AArch64 on GitHub [4]. Running the PoC revealed no conclusive
successes for the v3 exploit, which should mean that our cores are also
not vulnerable to Spectre v2.

Variants 1 and 4 was not considered because Arm's documentation states
that v1 and v4 mitigations are completely unrelated to those for v2 [5].

All PoC runs were conducted within a regular Android app's context with
the app's processes locked to the big and Prime clusters (CPUs 4-7),
since Arm states that the little cluster's cores (Cortex-A55) are
not affected by any variants of Spectre [2].

[1] https://en.wikichip.org/wiki/qualcomm/snapdragon_800/855
[2] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Security%20update%2010%20September%2018/Kernel_Mitigations_Detail_v1.7.pdf?revision=730b8541-ca91-4fde-a2bb-4093054748ae
[3] https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability
[4] https://github.com/lgeek/spec_poc_arm
[5] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Security%20update%2010%20September%2018/Kernel_Mitigations_Detail_v1.7.pdf?revision=730b8541-ca91-4fde-a2bb-4093054748ae

Signed-off-by: Danny Lin <danny@kdrag0n.dev>
Change-Id: I4411899b7da9a7e1899ea7532e922c40bb077ab1

arch/arm64/configs/vendor/pixel_experience-a52q_defconfig

@@ -640,7 +640,7 @@ CONFIG_FORCE_MAX_ZONEORDER=11
 # CONFIG_UNMAP_KERNEL_AT_EL0 is not set
 CONFIG_HARDEN_BRANCH_PREDICTOR=y
 # CONFIG_PRINT_VMEMLAYOUT is not set
-CONFIG_ARM64_SSBD=y
+# CONFIG_HARDEN_BRANCH_PREDICTOR is not set
 CONFIG_ARM64_TAGGED_ADDR_ABI=y
 CONFIG_ARMV8_DEPRECATED=y
 CONFIG_SWP_EMULATION=y

arch/arm64/configs/vendor/pixel_experience-a72q_defconfig

@@ -637,7 +637,7 @@ CONFIG_FORCE_MAX_ZONEORDER=11
 # CONFIG_UNMAP_KERNEL_AT_EL0 is not set
 CONFIG_HARDEN_BRANCH_PREDICTOR=y
 # CONFIG_PRINT_VMEMLAYOUT is not set
-CONFIG_ARM64_SSBD=y
+# CONFIG_HARDEN_BRANCH_PREDICTOR is not set
 CONFIG_ARM64_TAGGED_ADDR_ABI=y
 CONFIG_ARMV8_DEPRECATED=y
 CONFIG_SWP_EMULATION=y