ARM: dts: keystone-k2g-evm: Add QSPI DT node.

66AK2G EVM has a s25fl512s flash connected to QSPI. Add DT nodes for the
same.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>