config: Disable CONFIG_UNMAP_KERNEL_AT_EL0

This was introduced and enabled with the Sony 47.1.A.12.179 kernel and introduces kernel page-table isolation (KPTI) aka KAISER to mitigate Spectre variant 3 (aka Meltdown).

According to ARM, only the Cortex-A75 (as found in SD845) is affected and not our Cortex-A73 (as found in SD835).