irqchip/gic-v3-its: Narrow down Entry Size when used as a divider

GITS_BASER<n>'s Entry Size is much smaller than 64-bit, but when it
used as a divider it forces compiler to generate __aeabi_uldivmod if
build in 32-bit mode. So, casting it to int (like it is done in other
places) where used as a divider would give a hint to compiler that
32-bit division can be used.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>