sdhci: Add get_{max,timeout}_clock callbacks

Some controllers do not provide clock information in their capabilities
(in the Samsung case, it is because there are multiple clock sources
available to the controller). Add hooks to allow the system to supply
clock information.

p.s.
In the original Ben's patch there was a bug that makes sdhci_add_host()
return -ENODEV even if callbacks were specified. This is fixed now.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>