arm64: dts: lx2160a: add FlexSPI node property

Add fspi node property for LX2160A SoC for FlexSPI driver.
Property added for the FlexSPI controller and for the connected
slave device for the LX2160ARDB target.
This is having two SPI-NOR flash device, mt35xu512aba, connected
at CS0 and CS1.

Signed-off-by: Yogesh Narayan Gaur <yogeshnarayan.gaur@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>