mmc: fsl_esdhc: Allow all supported prescaler values

On i.MX, SYSCTL.SDCLKFS may be set to 0 in order to make the SD clock
frequency prescaler divide by 1 in SDR mode. In DDR mode, the prescaler
can divide by up to 512. Allow both of these settings.

The maximum SD clock frequency in High Speed mode is 50 MHz. On i.MX25,
this change makes it possible to get 48 MHz from the USB PLL
(240 MHz / 5 / 1) instead of only 40 MHz from the USB PLL
(240 MHz / 3 / 2) or 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>