SUPPORT.MD: Correct the amount of physical memory supported for Arm

As part of XSA-385, SUPPORT.MD gained a statement regarding the amount
of physical memory supported.

However, booting Xen on a Arm platform with that amount of memory would
result to a breakage because the frametable area is too small.

The wiki [1] (as of April 2022) claims we were able to support up to
5 TiB on Arm64 and 16 GiB. However, this is not the case because
the struct page_info has always been bigger than expected (56 bytes
for 64-bit and 32-bytes for 32-bit).

I don't have any HW with such amount of memory. So rather than
modifying the code, take the opportunity to use the limit that should
work on Arm (2 TiB for 64-bit and 12 GiB for 32-bit).

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com> #arm part

SUPPORT.md

@@ -50,9 +50,10 @@ For the Cortex A57 r0p0 - r1p1, see Errata 832075.
 
 ### Physical Memory
 
-    Status: Supported up to 8 TiB
-
-Hosts with more memory are supported, but not security supported.
+    Status, x86: Supported up to 8 TiB. Hosts with more memory are
+                 supported, but not security supported.
+    Status, Arm32: Supported up to 12 GiB
+    Status, Arm64: Supported up to 2 TiB
 
 ### Physical Memory Hotplug