arm64: zynqmp: Fix zynqmp mini qspi max frequency

For zynqmp qspi, frequencies up to 40MHz will work irrespective
of feedback clock enabled or disabled. If we want higher than
40Mhz the feedback clock should be enabled.

With spi-max-frequency 108MHz it is not working when the feedback
clock is disabled. Change it to 40MHz so that it works irrespective
of feedback clock enabled or disabled.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Acked-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>