ARM: dts: am335x-myirtech: Add missing NAND wait pin definition

The NAND wait pin is connected to the GPMC, but this is not mentioned
in the DT bindings. Let's fix this.

Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Message-Id: <20230524091157.775960-1-eagle.alexander923@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>