RAS: Introduce handler for EL3 EAs

External Aborts while executing in EL3 is fatal in nature. This patch
allows for the platform to define a handler for External Aborts received
while executing in EL3. A default implementation is added which falls
back to platform unhandled exception.

Change-Id: I466f2c8113a33870f2c7d2d8f2bf20437d9fd354
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>