pinctrl: meson: meson8: rename the "gpio" function to "gpio_periphs"

Rename the existing "gpio" function to "gpio_periphs". This makes it
consistent with the "gpio_aobus" function. Also GXBB and GXL are also
using the "gpio_periphs" naming, so this makes the code here consistent
with other Amlogic pinctrl drivers.

No functional changes since thee "gpio" function is currently not used.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>