mmc: sdhci-esdhc-imx: add std tuning support for mx6sl

The mx6sl supports standard sdhci tuning, then esdhc_executing_tuning
is only needed for mx6q/dl. We introduce is_imx6_usdhc() and
is_imx6sl_usdhc() to handle the difference.

The standard tuning is enabled by setting ESDHC_TUNE_CTRL_STD_TUNING_EN bit
in new register ESDHC_TUNE_CTRL and operates with new tuning bits
defined in SDHCI_ACMD12_ERR register.

Note: mx6sl can also work on the old manually tuning mode as mx6q/dl if
not enable standard tuning mode.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>