mmc: sdhci: add hooks for platform specific tuning

The tuning of some platforms may not follow the standard host control
spec v3.0, e.g. Freescale uSDHC on i.MX6Q/DL.
Add a hook here to allow execute platform specific tuning instead of
standard host controller tuning.

The hook only replaces the tuning process, so it's placed after tuning
checking and before the real tuning process.

Some notes for the tuning hook:
1) it needs handle lock itself if it wants to access host controller
according platform specific implementation.
2) do not need to handle runtime pm since it executes with runtime pm
get already.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>