uboot-mediatek: u7623: remove keys pin function

uboot-mediatek: u7623: remove keys pin function

Apparently U-Boot will discard whole node if requested pin function is
unknown to the driver. This resulted in inability to interact with
U-Boot on the said board, as U-Boot always assumed the recovery key
pressed and issued recovery procedure. Log snippet:

button_gpio gpio-keys: pinctrl_select_state_full: pinctrl_config_one: err=-38
reset button found
button pushed, resetting environment

Recovery procedure also booted recovery image, which didn't affect much
the 23.05.x release, since the root fs argument was valid, so changes
persisted. But as 24.10.x hit with fitblk, the board will boot only
recovery image (initramfs) because of default bootargs will reset on each
boot and U-Boot provided bootargs took precedence.

Fixes: 42eeb22450f2 ("uboot-mediatek: fix factory/reset button")
Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Link: https://patchwork.ozlabs.org/project/openwrt/patch/20250304164507.60511-1-tmn505@terefe.re/
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>