arm: gic: Register irqdomain after adding it

Recently a change was added to break apart irq_domain_add() into
two functions. The first creates the domain, and the second
registers it.

This change abides to this new interface and registers the
GIC irq_domain after creating it.

Change-Id: I34c6a1f56dc27113a9b72f0fefa577276f91bbc8
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>