ArmPkg/ArmMmuLib: Introduce region types for RO/XP WB cached memory

To prepare for the enablement of booting EFI with the SCTLR.WXN control
enabled, which makes all writeable memory regions non-executable by
default, introduce a memory type that we will use to describe the flash
region that carries the SEC and PEIM modules that execute in place. Even
if these are implicitly read-only due to the ROM nature, they need to be
mapped with read-only attributes in the page tables to be able to
execute from them.

Also add the XP counterpart which will be used for all normal DRAM right
at the outset.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>

ArmPkg/Include/Library/ArmLib.h

@@ -35,6 +35,12 @@ typedef enum {
   // Do NOT use below two attributes if you are not sure.
   ARM_MEMORY_REGION_ATTRIBUTE_WRITE_BACK_NONSHAREABLE,
 
+  // Special region types for memory that must be mapped with read-only or
+  // non-execute permissions from the very start, e.g., to support the use
+  // of the WXN virtual memory control.
+  ARM_MEMORY_REGION_ATTRIBUTE_WRITE_BACK_RO,
+  ARM_MEMORY_REGION_ATTRIBUTE_WRITE_BACK_XP,
+
   ARM_MEMORY_REGION_ATTRIBUTE_WRITE_THROUGH,
   ARM_MEMORY_REGION_ATTRIBUTE_DEVICE,
 } ARM_MEMORY_REGION_ATTRIBUTES;