irqchip/gicv3-its: Split its_alloc_tables() into two functions

irqchip/gicv3-its: Split its_alloc_tables() into two functions

The function is getting out of control, it has too many goto
statements and would be too complicated for adding a feature
two-level device table. So, it is time for us to cleanup and
move some of the logic to a separate function without affecting
the existing functionality.

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