feat(runtime): add separate stack for exception handler

This commit updates the Synchronous exception handler to use SP_EL0.
This helps if the exception happens due to SP_EL2 overflow.

Details:
 - Save SP_EL0 in the rec structure
 - Add a new mm region to the high region for the handler stack
 - Save/restore SP_EL0 on Realm exit/enter

Change-Id: Ie230dd24a27766a092798f412262e2a08cd831f4
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>