feat(plat/arm/sgi): increase memory reserved for bl31 image

Increase the size of bl31 image by 52K to accomodate increased size of
xlat table.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: Ic3a8d8be1104adf48d22aa829e2197f710b6b666