xilinx: Add jedec compatible string for QSPI (Zynq/ZynqMP)

Add missing "jedec, spi-nor" compatible string for QSPI flash node.
Spi-nor framework uses this compatibility string to probe &
initialize flash. With missing compatibility string we are observing
below error:

Zynq> sf probe 0 0 0
 jedec_spi_nor spi_flash@0:0: unrecognized JEDEC id bytes:
00, 00, 00  Failed to initialize SPI flash at 0:0 (error -2)

Signed-off-by: Raju Kumar Pothuraju <raju.kumar-pothuraju@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>