mailbox: pcc: Consolidate subspace interrupt information parsing

Extended PCC subspaces(Type 3 and 4) differ from generic(Type 0) and
HW-Reduced Communication(Type 1 and 2) subspace structures. However some
fields share same offsets and same type of structure can be use to extract
the fields. In order to simplify that, let us move all the IRQ related
information parsing into pcc_parse_subspace_irq and consolidate there.
It will be easier to extend it if required within the same.

Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>