feat(mmc): manage SD Switch Function for high speed mode

On SD-cards, Switch Function Command (CMD6) is used to switch
functions, like setting High Speed mode. It is useful for high capacity
cards to double frequency (from 25MHz by default to 50MHz).
If the SD-card is High Capacity, a CMD6 is issued after filling the
device information. If High Speed mode is supported and the switch is
OK, then the max_bus_freq can be set to 50MHz. The driver set_ios()
function should then be called to update peripheral configuration,
especially clock prescaler.

Change-Id: I2d6807aa7f9440d2b2f907a747cd3b47a2ba1545
Signed-off-by: Yann Gautier <yann.gautier@st.com>