pinctrl: single: check "register-width" DT property

In more recent versions of the Linux kernel the driver's probe function
returns an error if the "pinctrl-single,register-width" DT property is
missing. The lack of this information, in fact, does not allow to know
whether to access the registers of the controller at 8, 16, ... bits.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>