mediatek: drop pwm7_pins from Buffalo WSR-2533DHP2

mediatek: drop pwm7_pins from Buffalo WSR-2533DHP2

MediaTek MT7622 doesn't support ch7 of PWM and pinctrl groups were dropped from
driver source[0]. So pwm7-related groups are unavailable now, then, kernel shows a
warning.

[    0.370264] mt7622-pinctrl 10211000.pinctrl: invalid group "pwm_ch7_2" for function "pwm"

Drop that pinmux from pinctrl node.

[0]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/pinctrl/mediatek/pinctrl-mt7622.c?id=57972641810a97566ffd13e4be3f6a66d61eb3b5

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
(cherry picked from commit 4d9113017b2d401e821fb008518cdd660dafb8a3)