ARM: 6807/1: realview: Fix secondary GIC initialisation for EB with MPCore tile

The second GIC, present when EB board is used with a MPCore tile,
was initialised starting with irq number 64, which made interrupts
64-95 in the primary GIC unusable.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>