mmc: fsl: reset to normal boot mode when eMMC fast boot

When booting in eMMC fast boot, MMC host does not exit from
boot mode after bootrom loading image. So the first command
'CMD0' sent in uboot will pull down the CMD line to low and
cause errors.

This patch cleans the MMC boot register in "mmc_init" to put the
MMC host back to normal mode.

Also clear DLL_CTRL delay line settings at USDHC initialization
to eliminate the pre-settings from boot rom.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>