irqchip/gicv3-its: Avoid cache flush beyond ITS_BASERn memory size

Function its_alloc_tables() maintains two local variables, "order" and
and "alloc_size", to hold memory size that has been allocated to
ITS_BASEn. We don't always refresh the variable alloc_size whenever
value of the variable order changes, causing the following two
problems.

  - Cache flush operation with size more than required.
  - Information reported by pr_info is not correct.

Use a helper macro that converts page order to size in bytes instead of
variable "alloc_size" to fix both the problems.

Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>