pinctrl: tegra: Use correct offset for pin group

Function tegra_pinctrl_gpio_request_enable() and
tegra_pinctrl_gpio_disable_free() uses pin offset instead
of group offset, causing the driver to use wrong offset
to enable gpio.

Add a helper function tegra_pinctrl_get_group() to parse the
pin group and determine correct offset.

Signed-off-by: Kartik K <kkartik@nvidia.com>
Signed-off-by: Prathamesh Shete <pshete@nvidia.com>
Link: https://lore.kernel.org/r/20211025110959.27751-1-pshete@nvidia.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>