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

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