i.MX25: esdhc: Add mxc_get_clock infrastructure

Defining CONFIG_FSL_ESDHC brings in a call to get_clocks, so let's
implement get_clocks function. This is how it seems to be implemented
elsewhere.

Signed-off-by: Timo Ketola <timo@exertus.fi>
Acked-by: Stefano Babic <sbabic@denx.de>