feat(sgi): configure SRAM and BL31 size for sgi platform

Update SRAM size for Neoverse reference design platforms from 256KB to
512KB. This is required to place and execute BL31 image from the
on-chip SRAM. Additionally, revise BL31 image size to accommodate
larger BL31 images of multi-chip platforms.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: I11c2672a1089f24a9fafcf6555b8e1d52032cfde