mx6: clock: Pass the frequency as argument of enable_fec_anatop_clock()

Provide an argument to enable_fec_anatop_clock() to specify the clock frequency
that will be generated.

No changes are made to mx6slevk, which uses the default 50MHz fec clock.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>