Do not make symbols for enums in gic(v3)

These conflict with each other cause cause a multiple definition error
with clang 11.

Signed-off-by: Stefan O'Rear <sorear@fastmail.com>

trivial: Fix compiler warnings

- Mark unused variables unused
- Initialize uninitialized variables

libplatsupport: Add GICv3 IRQ chip parser

This new parser module is able to parse interrupts under the format
specified by the GICv3 bindings. For now, the parser is only able to
parse SPI interrupts and not PPI interrupts.