ARM: gic: add missing distributor defintions

Add missing register map offsets for the distributor and rename
GIC_DIST_ACTIVE_BIT to GIC_DIST_ACTIVE_SET to be consistent.

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>