mmc: omap_hsmmc: use a default 52MHz max clock rate if none is specified

mmc_of_parse() doesn't set a default value if none is available in DT.
In that case, use a default 52MHz clock rate.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Tested-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>