pinctrl: rockchip: rk3588: Fix support for rockchip_get_mux()

GPIO IOMUX control is located at PMU2_IOC or BUS_IOC offset on RK3588.

Based on Linux commit fdc33eba11c5 ("pinctrl/rockchip: add rk3588
support").

Compared to the Linux commit, this include a fix so that the iomux of
GPIO0_B4-D7 is reported correctly.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>