Platform/ARM: Reduce System Memory Size for FVP with RME extensions

For older FVPs (without support for RME extension) the top 16MB of
DRAM1 is reserved as Trusted DRAM. However, the latest FVP Base RevC
AEM Model [1] has support for RME extension. When RME extension is
present the top 64MB of DRAM1 (i.e. at the top of the 32bit address
space) is carved out for four-world support in TF-A [2].

Therefore, reduce the System Memory size by 64MB.

Reference:
[1] FVP Base RevC AEM Model (available on x86_64 / Arm64 Linux)
(https://developer.arm.com/Tools%20and%20Software/
Fixed%20Virtual%20Platforms)

[2] commit c872072 (https://review.trustedfirmware.org/plugins/gitiles/


TF-A/trusted-firmware-a/+/c8720729726faffc39ec64f3a02440a48c8c305a))

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>