Fix a rk356x pinctrl register offset

The pull-up/pull-down register offset was wrong on the Rockchip rk356x.
It was set such that the driver would modify the IOMUX control registers.

This seems to work with the current device tree files, but fails with
upstream files. Fix the offset so the later calculation has the correct
offset for the pull-up/pull-down control register.

Sponsored by: The FreeBSD Foundation