ARM: add missing HYP mode constant

In order to be able to use the various mode constants (far more
readable than random hex values), add the missing HYP and A
values.

Also update arm/lib/interrupts.c to display HYP instead of an
unknown value.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>