arm64: Kconfig.platforms: fix warning unmet direct dependencies

When ARCH_MXC get enabled, ARM64_ERRATUM_845719 will be selected and
this warning will happen when COMPAT isn't set.

WARNING: unmet direct dependencies detected for ARM64_ERRATUM_845719
  Depends on [n]: COMPAT [=n]
  Selected by [y]:
  - ARCH_MXC [=y]

Rework to add 'if COMPAT' before ARM64_ERRATUM_845719 gets selected,
since ARM64_ERRATUM_845719 depends on COMPAT.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>