mmc: sdhci-esdhc: use writel/readl as general APIs

Add one flag to indicate the GPIO CD/WP is enabled or not
on imx platforms, and reuse the writel/readl as the general
APIs for imx SOCs.

Signed-off-by: Richard Zhu <Hong-Xing.Zhu@freescale.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>