mmc: dw_mmc: add support for implementation specific callbacks

The core dw-mshc controller driver can let platform specific
implementations of the dw-mshc controller to control the hardware
as required by such implementations. This is acheived by invoking
implementation specific (optional) callbacks. Define the list of
callbacks supported the add invocation points for the same.

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>