ARM: OMAP: Remove unnecessary clk structure

In the plat/dmtimer.h there is a structure named "clk" declared. This structure
is not used and appears to be left over from previous code. Hence, remove this
unused structure.

Verified that both omap1 and omap2plus kernel configurations build with this
change.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>