drivers: gpio: xlnx_ps: fix up initialization priorities

Current setup tries to initialize the bank driver before the parent one,
which is the inverse of what the devicetree hierarchy implies and
causes a bunch of:

ERROR: /soc/gpio@e000a000/psgpio_bank@3 PRE_KERNEL_1 40 32 <
/soc/gpio@e000a000 PRE_KERNEL_2 40 10

Change the bank driver to initialize at PRE_KERNEL_1 as the parent
drivers so that ordinals take care of priority between these two.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>