ITS: Increase stack size

This patch increases stack size for internal trusted
storage service.
This is the minimum stack size for running all the regression
tests for different configurations with SST_RAM_FS disabled on
Musca-b1.
And also removes the force setting of SST_RAM_FS for Musca-b1.

Change-Id: Ib73e1bed5bd9108681d650859ea87b200a0ae2e5
Signed-off-by: Kevin Peng <kevin.peng@arm.com>