MdePkg/Base: introduce MAX_ALLOC_ADDRESS

On some architectures, the maximum representable address deviates from
the virtual address range that is accessible by the firmware at boot
time. For instance, on AArch64, UEFI mandates a 4 KB page size, which
limits the address space to 48 bits, while more than that may be
populated on a particular platform, for use by the OS.

So introduce a new macro MAX_ALLOC_ADDRESS, which represent the maximum
address the firmware should take into account when allocating memory
ranges that need to be accessible by the CPU at boot time.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Include/Arm/ProcessorBind.h

@@ -148,6 +148,11 @@ typedef INT32   INTN;
 ///
 #define MAX_ADDRESS  0xFFFFFFFF
 
+///
+/// Maximum usable address at boot time
+///
+#define MAX_ALLOC_ADDRESS   MAX_ALLOC_ADDRESS
+
 ///
 /// Maximum legal ARM INTN and UINTN values.
 ///