dmaengine: edma: fix build without CONFIG_OF

During the edma rework, a build error was introduced for the
case that CONFIG_OF is disabled:

drivers/built-in.o: In function `edma_tc_set_pm_state':
:(.text+0x43bf0): undefined reference to `of_find_device_by_node'

As the edma_tc_set_pm_state() function does nothing in case
we are running without OF, this adds an IS_ENABLED() check
that turns the function into an empty stub then and avoids the
link error.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: ca304fa9 ("ARM/dmaengine: edma: Public API to use private struct pointer")
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>