pinctrl: mediatek: remove unused drv_offset field

pinctrl: mediatek: remove unused drv_offset field

The 'drv_offset' member in 'struct mtk_pinctrl_devdata' has been unused
since the driver's inception. Drive strength control for MediaTek
pinctrl drivers is actually configured via 'pin_drv_grp',
making this specific offset field redundant.

Remove the unused field from the common 'mtk_pinctrl_devdata' structure
and its corresponding initialization in the mt8365 driver to clean up
the code and avoid confusion.

Signed-off-by: Akari Tsuyukusa <akkun11.open@gmail.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>