MLK-11370 dts: imx6ul-evk: enhance driver strength of sd clk signal

The SD CLK signal measured by scope shows the signal rise/fall time
does not meet the spec requirement, need increase the pad driver
strength to meet the spec.
The pad setting value is given by the HW team which has been verified
from scope.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
(cherry picked from commit e4944a51c4bb950a8f13e421eefc000a87350e23)