pwm: omap-dmtimer: Allow compiling with COMPILE_TEST

The dependency on OMAP_DM_TIMER is only a runtime dependency. Also
OMAP_DM_TIMER cannot be enabled without ARCH_OMAP being enabled.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>