arm_boot: Pass base address of GIC CPU interface, not whole GIC

arm_boot: Pass base address of GIC CPU interface, not whole GIC

The arm_boot secondary boot loader code needs the address of
the GIC CPU interface. Obtaining this from the base address
of the private peripheral region was possible for A9 and 11MPcore,
but the A15 puts the GIC CPU interface in a different place.
So make boards pass in the GIC CPU interface address directly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>