gpio: uniphier: rework single device node model

First, I implemented this driver as per-bank model, but it was
a design mistake.

  - There are 31 banks in the maximum case.  It is painful to add
    so many nodes to DT.

  - The IRQ control registers are shared between banks.  Per-bank
    design is a problem for Linux.  The counterpart for Linux turned
    around to the single node model.

Rework based on the driver for Linux.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>