ARM: dts: Unify pwm-omap-dmtimer node names

There is no reg property for pwm-omap-dmtimer.

Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>