arm: zynq: dts: add spi flash node to zedboard

Add a flash node to fix the detection of the memory IC.
With the changes introduced with commit 8fee8845
("enf_sf: reuse setup_flash_device instead of open coding it")
the SPI speed is now read from device-tree or a default value
is applied. This replaced the old behavior of setting the
SPI speed to CONFIG_ENV_SPI_MAX_HZ.

As this board didn't have a flash node, the default value
was applied to the SPI speed, producing an error when probing
the flash memory (speed too slow).

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>