ARM: dts: keystone-k2g-evm: add bindings for SPI NOR flash

K2G EVM has n25q128a13 SPI NOR flash on SPI1. Enable SPI1 in the DT
node as well as add a subnode for the SPI NOR.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>