ARM: OMAP: Add enable/disable functions for dmtimer

Add enable/disable functions which effectively control the GPT iclk and fclk.

Signed-off-by: Timo Teras <timo.teras@solidboot.com>
Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>