ARM: OMAP: Use GPT iclk only when needed

This patch makes the OMAP2 dmtimers module using the interface
clocks only while the registers are accessed (except GPT1 which has
iclk enabled all the time).

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