ArmPkg/AArch64Mmu: use correct AP[] bits in ArmClearMemoryRegionReadOnly

ArmPkg/AArch64Mmu: use correct AP[] bits in ArmClearMemoryRegionReadOnly

The function ArmClearMemoryRegionReadOnly() was supposed to undo the
effect of ArmSetMemoryRegionReadOnly(), but instead, it sets the permissions
to EL0-no access, EL1-read-only. Since the EL0 bit should be 1 to align
with EL2/3 (where the bit is SBO), use TT_AP_RW_RW instead, which makes the
entry read-write for EL0 when executing at EL1, and read-write for all other
levels.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>