MLK-22179-2 imx8mn_evk: Enable FSPI DQS loopback for high freq

MLK-22179-2 imx8mn_evk: Enable FSPI DQS loopback for high freq

There is an divider on imx8mn will always divide 2 to flexspi root clock.
So actual SCLK output to device is 50Mhz on imx8mn not 100Mhz.

After changing the root clock setting to configure SCLK to 100Mhz, found
the read data is not correct. Must enable the internal DQS pad loopback
to fix the problem.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit 9ff3ae4f9ebbb81bd42d81729cec4525c6e9b33e)