ARM: dts: add rk3288 i2s controller

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>