arm64: dts: ls1028a: Add FlexSPI support

Add fspi node property for LS1028A SoC for FlexSPI driver.
Property added for FlexSPI controller and for the connected
slave device for the LS1028ARDB and LS1028AQDS target.
RDB and QDS are having one SPI-NOR flash device, mt35xu02g
connected at CS0.
This flash device "mt35xu02g" is tested for octal read

Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Ashish Kumar <Ashish.Kumar@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>