sh-pfc: Rename gpio arguments to be consistent with the rest of the code

The gpio_get_data_reg() and gpio_setup_data_reg() functions both take an
argument named gpio. The argument contains a GPIO offset for the first
function and a pin index for the second one. Rename them to offset and
idx respectively to match the rest of the code.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Yusuke Goda <yusuke.goda.sx@renesas.com>