mmc: sdhci-esdhc-imx: add imx6sx support

The imx6sx usdhc is derived from imx6sl, the difference is minor.
imx6sx have the errata ESDHC_FLAG_ERR004536 fixed.
So introduce a new compatible string for imx6sx to distinguish them.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>