sh: pfc: Add gpio_read_bit() for data register access

Introduce gpio_read_bit() for data register read access
and modify sh_gpio_get_value() to make use of the new
function instead of gpio_read_reg(). The purpose of
this change is to update the code to only use the
gpio_read_reg() function for config register access.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>