ARM: OMAP2+: dmtimer: cleanup fclk usage

With omap_hwmod_get_main_clk() now available, this can be passed to
clk_get() to extract the fclk and thus avoid construction of fclk name.
Corrected the timer fck name mis-match between clock44xx_data.c and
omap_hwmod_44xx_data.c. For other platforms this is already taken care.

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