mx25pdk: Set the eSDHC PER clock to 48 MHz

mx25pdk: Set the eSDHC PER clock to 48 MHz

The maximum SD clock frequency in High Speed mode is 50 MHz. This change
makes it possible to get 48 MHz from the USB PLL (240 MHz / 5 / 1)
instead of the previous 33.25 MHz from the AHB clock (133 MHz / 2 / 2).

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>