pinctrl: qcom: spmi-gpio: implement .get_direction()

GPIO controller driver should typically implement the .get_direction()
callback as GPIOLIB internals may try to use it to determine the state
of a pin. Since introduction of shared proxy, it prints a warning splat
when using a shared spmi gpio.

The implementation is not easy because the controller supports enabling
the input and output logic at the same time, so we aligns on the
behaviour of the .get() operation and return -EINVAL in other
situations.

Fixes: eadff3024472 ("pinctrl: Qualcomm SPMI PMIC GPIO pin controller driver")
Fixes: d7b5f5cc5eb4 ("pinctrl: qcom: spmi-gpio: Add support for GPIO LV/MV subtype")
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>