dmaengine/ARM: omap-dma: Fix the DMAengine compile test on non OMAP configs

The DMAengine driver for omap-dma use three function calls from the
plat-omap legacy driver. When the DMAengine driver is built when ARCH_OMAP
is not set, the compilation will fail due to missing symbols.
Add empty inline functions to allow the DMAengine driver to be compiled
with COMPILE_TEST.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>