ARM: OMAP2+: hwmod: add omap_hwmod_get_main_clk() API

Add an API to get main clock name associated with a given @oh.
This will avoid the need to construct fclk names during early
initialization in order to get fclk handle using clk_get().

Signed-off-by: Tarun Kanti DebBarma <tarun.kanti@ti.com>
Cc: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Kevin Hilman <khilman@ti.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>