ENGR00155612-1 [mx6q]dynamically sd pad setting change

ENGR00155612-1 [mx6q]dynamically sd pad setting change

on mx6q, it supports sd3.0 card with DDR 50MHz, SDR 100Mhz and SDR 200MHz.
sd pads have to be changed dynamically for these large scale of clock
frequencies.
add different pad setting definitions for these clock frequencies under
board file, since these settings are really board dependent.
add callback funtion in sdhc platform data to give driver approach to
change pad setting according to current clock frequency.

Signed-off-by: Tony Lin <tony.lin@freescale.com>