irqchip: gicv3-its: Use 64KB page as default granule

The field of page size in register GITS_BASERn might be read-only
if an implementation only supports a single, fixed page size. But
currently the ITS driver will throw out an error when PAGE_SIZE
is less than the minimum size supported by an ITS. So addressing
this problem by using 64KB pages as default granule for all the
ITS base tables.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
[maz: fixed bug breaking non Device Table allocations]
Signed-off-by: Yun Wu <wuyun.wu@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1425659870-11832-9-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>