pinctrl: single: search for the bits property when parsing bits

The pcs_parse_bits_in_pinctrl_entry function should search
for the "pinctrl-single,bits" and not "pinctrl-single,pins"

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>