irqchip/gicv3-its: ITS table size should not be smaller than PSZ

When allocating a device table, if the requested allocation is smaller
than the default granule size of the ITS then, we need to round up to
the default size.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
[ stuart: Added comments and massaged changelog ]
Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Reviewed-by: Marc Zygnier <marc.zyngier@arm.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/1432134795-661-1-git-send-email-stuart.yoder@freescale.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>