rockchip: rk3588-generic: Use M0 iomux for UART2

The default pinctrl for UART2 is using M1 iomux, however, all other
supported RK3588 boards in U-Boot use M0 iomux for UART2.

Change to use the M0 pinctrl for UART2 to match other supported boards.

Fixes: b54c3d0d ("rockchip: board: Add minimal generic RK3588S/RK3588 board")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>