ENGR00295564 mmc: sdhci-esdhc-imx: no need busfreq for imx6qdl

ENGR00295564 mmc: sdhci-esdhc-imx: no need busfreq for imx6qdl

The usdhc of i.MX6Q/DL can work well under low power mode without
request high bus freq. So we do not need request bus freq for i.MX6Q/DL.
It can save power for i.MX6D/DL due to it saves a lot busfreq switch
cost as well as the CPU time runing on high bus freq after switch
during low power mode.

A new flag ESDHC_FLAG_BUSFREQ is added to indicated this requirement.
Currently only i.MX6SL is using it.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
(cherry picked from commit 075196777d00bf9507d68a76bf25f6c7e776102f)