ARM: OMAP: Update dmtimers

- Initialize timer outside of spinlock to reduce the time the spinlock is held
- Do clk_get to the source clocks during initialization to avoid sleeping later
- New function to set counter register

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>