Yes, correct for i.MX6 there is no max. clock settings because
every ARM sub-arch has a different, incompatible struct clk implementation.
Seems you can add such max. restriction in function _clk_usdhc_round_rate
(clock.c file).
More about Linux clock architecture one can find on eLinux.org:
"Common clock framework: how to use it" - eLinux.org
thanks