MIPS: lantiq: drop ltq_gpio_request() and gpio_to_irq()

MIPS: lantiq: drop ltq_gpio_request() and gpio_to_irq()

As part of the conversion to OF we also implement pinctrl drivers. Previously
we used ltq_gpio_request() to set pinmuxing. This is now obselete and we can
hence drop the function.

Additionally we remove gpio_to_irq() from the gpio driver and move it to a
header file.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3801/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>