arm64: zynqmp: Add spi-flash compatible string to flash node

spi-flash compatible string is needed for reading tx and rx bus
widths, hence add this compatible string to flash node.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>