feat(fvp): scale SP_MIN max size based on SRAM size

The maximum size for SP_MIN in the FVP is currently
fixed and does not scale with the SRAM size.
This update adjusts the SP_MIN size according to
the SRAM size used to build the FVP platform.

Change-Id: I95527e8ae6f8a73c336ed4fe05ace5de86d8991d
Signed-off-by: Ryan Everett <ryan.everett@arm.com>