pinctrl: pinctrl-single: Add 'pinctrl-single, bits' support

The TI Davinci (da850/l138/am1808) use pinctrl-single,bits for
pinmuxing peripherals.  This patch allosw the pinctrl-single
driver to parse the pinctrl-single,bits options and correctly
setup devices.

Signed-off-by: Adam Ford <aford173@gmail.com>