ArmPlatformPkg/FVP: map motherboard VRAM as uncached memory

ArmPlatformPkg/FVP: map motherboard VRAM as uncached memory

The VRAM of the PL111 on the FVP Base/Foundation models is described as
device memory rather than uncached memory, which is not an accurate
description of the nature of the region (i.e., a framebuffer), and may
result in problems when using accelerated string routines to access the
region, since this may legally involve unaligned accesses or DC ZVA
instructions, which are not allowed on device mappings.

So split of the 8 MB VRAM region into a separate region, and map it using
memory attributes.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>