mmc: dw_mmc: prepare functions in dw_mmc-pltfm for reuse

Platform implementations of dw-mshc controller may choose to extend the
features of the standard dw-mshc controller such as adding additional
clocking options or modifying the bus interface. Support for such
implementation specific extensions can be incorporated into dw_mmc-pltfm,
but including multiple such platform specific implementations would
convolute the existing dw_mmc-pltfm code.

Instead, it would be better to create implementation specific platform
drivers to support implementation specific features. Such platforms
drivers can reuse the existing infrastructure in dw_mmc-pltfm for
resource identification and controller registration and provide support
for implementation specific features. So, allow the infrastructure in
dw_mmc-pltfm to be reused by other implementation specific platform
drivers.

Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Will Newton <will.newton@imgtec.com>
Signed-off-by: Chris Ball <cjb@laptop.org>