fix(rdv3cfg1) Bumping up BL31 stack size to 0x700

Fix stack overflow in BL31 during CPU error injection by
increasing STACK_SIZE

The first CPU firmware first handling  FFA call consumes more
stack space than subsequent calls. This leads to corruption
of critical structures like the SDEI map and the RAS event map.
Increasing the STACK_SIZE in BL31 resolves this overflow and
ensures reliable error injection.

Signed-off-by: Glen Yeldho <glen.yeldho@arm.com>
Change-Id: I9bad7f9d12fba6d872ef07ce9d3f82683ec98d1a