mmc: sdhci-sprd: Add Spreadtrum's initial host controller

This patch adds the initial support of Secure Digital Host Controller
Interface compliant controller found in some latest Spreadtrum chipsets.
This patch has been tested on the version of SPRD-R11 controller.

R11 is a variant based on SD v4.0 specification.

With this driver, R11 mmc can be initialized, can be mounted, read and
written.

Original-by: Billows Wu <billows.wu@unisoc.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>