MLK-9501 dts: imx6sx-sdb: optimize usdhc3 pad settings

Detailed reproduce steps:
1. boot-up to Linux command prompt .
2. Plug SD3.0 UHS-I SD Card into SD3 Connector (make sure SD Card running
   at SD3.0 DDR50/1.8V).
2. write data to SD3 using "dd" command (SD3_CLK running at 1.8V/50MHz).
3. capture the SD3_CLK, SD3_DATA, SD3_CMD waveforms during data write using
   FET probe (>=1GHz)
4. CLK waveforms like triangular wave are observed.

HW team found that the pad setting of the SD3_CLK, SD3_DATA, SD3_CMD signal pins are
not optimized. In existing BSP, when running at SD3.0/DDR50/1.8V, SPEED/DSE/SRE
= 01/011/1 is used. They propose change it to -
 SD3_CLK: SPEED/DSE/SRE = 01/110/1.
 SD3_DATA/SD3_CMD: SPEED/DSE/SRE = 01/101/1.

SDHC high speed cards also had such issue(refer to MLK-9500).

We only changed the default state (<50Mhz) pad setting, for ultra high speed
state like 100Mhz and 200Mhz, it does not have such issue since they already
set to the maximum Drive Strength value.

Conflicts:

	arch/arm/boot/dts/imx6sx-sdb.dts

Signed-off-by: Dong Aisheng <b29396@freescale.com>
(cherry picked from commit 20ea8157d2d109a7b08e308e007e1eb3157689d4)
(cherry picked from commit 3b7935fb3e29526cb3fb0c06562f57e6eb8ad377)