mmc: do not overwrite cfg->f_max if "max-frequency" if missing

mmc_of_parse() in U-Boot is a pussy helper; it sets cfg->f_max to
52MHz even if DT does not provide "max-frequency" at all.  This can
overwrite cfg->f_max that may have been set to a reasonable default.

As the DT binding says, "max-frequency" is an optional property.
Do nothing if DT does not specify it.  This is the behavior of
mmc_of_parse() in Linux.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>