pinctrl: axp209: rename everything from gpio to pctl

This driver used to do only GPIO features of the GPIOs in X-Powers
AXP20X. Now that we have migrated everything to the pinctrl subsystem
and added pinctrl features, rename everything related to pinctrl from
gpio to pctl to ease the understanding of differences between GPIO
and pinctrl features.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>