spi: zynq_qspi: Add child pre probe function

Add child pre probe function in the driver. Update max_hz of priv from
spi_slave structure.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Link: https://lore.kernel.org/r/1657893679-20039-2-git-send-email-ashok.reddy.soma@xilinx.com


Signed-off-by: Michal Simek <michal.simek@amd.com>