mmc: fsl_esdhc: add pulse width detection workaround

HS400 mode on the LS1028A SoC isn't reliable. The linux driver has a
workaroung for the pulse width detection. Apply this workaround in
u-boot, too.

This will make HS400 mode work reliably on the LS1028A SoC.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>