pinctrl: pinctrl-single: Add pinctrl-single,bits type of mux

With pinctrl-single,bits it is possible to update just part of the register
within the pinctrl-single,function-mask area.
This is useful when one register configures mmore than one pin's mux.

pinctrl-single,bits takes three parameters:
<reg offset, value, sub-mask>

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
[Removed a misplaced comment]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>