ARM: dts: n900: remove unneeded unit name for dmtimer-pwm

This patch fixes the following DTC warnings for omap3-n900.dtb:

"dmtimer-pwm@9 has a unit name, but no reg property"

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>