ARM: OMAP2+: GPMC: Remove unused OneNAND get_freq() platform function

A platform function pointer for getting the frequency of a OneNAND device
was added so that a platform could specify a custom function for returning
the frequency and not just rely on the OneNAND version to determine the
frequency. However, this platform function pointer is not currently being
used and I am not sure if it ever has.

OneNAND devices are not so common these days and as far as I know not being
used with new devices. Therefore, it is most likely that this get_freq()
function pointer will not be used and so remove it.

Given that the get_freq() function pointer is not used, neither is the
clk_dep variable and so all references to it can also be removed.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Afzal Mohammed <afzal@ti.com>