gpio: dwapb: Add support for port B

The IP supports two ports, A and B, each providing up to 32 gpios.
The driver already creates a 2nd gpio bank by reading the 2nd node
from DT, so this is quite a simple change to support the 2nd bank.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>