mmc: Add a new callback function to enable/disable vdd

mmc: Add a new callback function to enable/disable vdd

Add a new callback function *set_vdd* which can be used
by the platform mmc driver to enable or disable vdd.
The mmc core can use *mmc_set_vdd* in order to invoke
the callback function. This will be used during power cycle
where the specification requires vdd to be disabled for
1ms and enabled again.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>