mmc: Add a new callback function to perform the 74 clocks cycle sequence

Add a new callback function *send_init_stream* which start a sequence of
at least 74 clock cycles.
The mmc core uses *mmc_send_init_stream* in order to invoke the callback
function. This will be used during power cycle where the specification
requires such a sequence after power up.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>