Platform: Switch to RAM FS for SST on MUSCA board

Set 'SST_RAM_FS' to 'ON' on the MUSCA_B1 and MUSCA_A boards to use the
RAM as a default device to store the FS in secure storage service.

Change-Id: Ib7ffaf6217e63a18cfe724cdcab23d3ce26aee0c
Signed-off-by: Edison Ai <edison.ai@arm.com>