dts: designware-gpio: remove reduplicative property bits

bits property indicates the number of in-use slots of available slots
for GPIOS. We have a similar property ngpios in gpio-controller.yaml,
we will use ngpios to calculate port_pin_mask. Let's remove bits and
only use ngpios.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>