mmc: fsl_esdhc: Fix i.MX53 eSDHCv3 clock

Commit 4f425280 ("mmc: fsl_esdhc: Allow all supported prescaler
values") made it possible to set SYSCTL.SDCLKFS to 0 in SDR mode on
i.MX, thus bypassing the SD clock frequency prescaler, in order to be
able to get higher SD clock frequencies in some contexts. However, that
commit missed the fact that this value is illegal on the eSDHCv3
instance of the i.MX53. This seems to be the only exception on i.MX,
this value being legal even for the eSDHCv2 instances of the i.MX53.

Fix this issue by changing the minimum prescaler value for the single
instance of the i.MX53 eSDHCv3 controller.

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