mmc: sdhci-esdhc-imx: add flag ESDHC_FLAG_USDHC

Add flag ESDHC_FLAG_USDHC to tell that the ESDHC is actually an USDHC
block, and replace the is_imx6q_usdhc() occurrences with inline function
esdhc_is_usdhc() which checks the flag.

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