Add dt for rk3288 i2s controller, since i2s clock pins and data pins
default to be GPIO, this patch also add pinctrl to mux them.

Tested on RK3288 board.
Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>