Add 16-bit single register pin controller support

Add 16-bit single register pin controller support

Enables the pinctrl-single driver to support 16-bit registers. Only
32-bit registers were supported previously. Reduced width registers are
required for some platforms, such as OMAP.

Signed-off-by: James Balean <james@balean.com.au>
Cc: Felix Brack <fb@ltec.ch>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Felix Brack <fb@ltec.ch>
Tested-by: Felix Brack <fb@ltec.ch>
Reviewed-by: Simon Glass <sjg@chromium.org>