ARM: 7318/1: gic: refactor irq_start assignment

The irq_start and hwirq_base assignment code is fairly hairy and ended
up being difficult to read following a conflict resolution for 3.2.

This patch rearranges the code slightly to make it easier to read.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>