arm64/acpi: disallow writeable AML opregion mapping for EFI code regions

arm64/acpi: disallow writeable AML opregion mapping for EFI code regions

Given that the contents of EFI runtime code and data regions are
provided by the firmware, as well as the DSDT, it is not unimaginable
that AML code exists today that accesses EFI runtime code regions using
a SystemMemory OpRegion. There is nothing fundamentally wrong with that,
but since we take great care to ensure that executable code is never
mapped writeable and executable at the same time, we should not permit
AML to create writable mapping.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Link: https://lore.kernel.org/r/20200626155832.2323789-3-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>