msm_otg: Set USB core clk to supported Max clk frequency

USB core clk is derived different ways on different platforms.
Sometime it is being derived from DFAB or AXI or having dedicated
clk. USB core can not tolerate any frequency change at runtime due
to hardware limitation. Hence it is must to set USB core clk to Max
supported clk frequency.

This change inquires Max supported clk frequency for USB Core clk
and request to set the same using Linux clock driver provided APIs.

Change-Id: I56780d4f8b4422efd2fca690d4b73c80f530444d
Signed-off-by: Mayank Rana <mrana@codeaurora.org>