ARM: OMAP: fix OMAP1 dmtimer build warning

Remove the OMAP1 version of omap_dm_timer_get_fclk(), and its associated
compile-time warning.  It would only BUG() if called, while it's only
called on OMAP2.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>