Improve readability of TFTF exceptions code

The only exceptions that TFTF expects are IRQs from the current
exception level. All other exceptions are unhandled and put the TFTF
into an irrecoverable state. Introduce an unhandled_exception macro
that makes it clearer.

Change-Id: Ia64600d540007a648c14d8663c91c60c8591eae6
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>