gpio: Use separate bitfield array to indicate GPIO is claimed

The current gpio-uclass design uses name field in struct gpio_dev_priv as
an indicator that GPIO is claimed by consumer. This overloads the function
of name field and does not work well for named pins not configured as GPIO
pins.

Introduce separate bitfield array as the claim indicator.

This unbreaks dual-purpose AF and GPIO operation on STM32MP since commit
2c38f7c3 ("pinctrl: pinctrl_stm32: Populate uc_priv->name[] with pinmux node's name")
where any pin which has already been configured as AF could no longer be
claimed as dual-purpose GPIO. This is important for pins like STM32 MMCI
st,cmd-gpios .

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>