add Cortex-A72 Spectre v4 (CVE-2018-3639) mitigation

[lilyanatia]

mitigate Spectre v4:

For Cortex-A57 and Cortex-A72:

• Set bit 55 (disable load pass store) of CPUACTLR_EL1 (S3_1_C15_C2_0).