dmaengine: imx-sdma: Add imx6sx platform support

The new Solo X has more requirements for SDMA events. So it creates
a event mux to remap most of event numbers in GPR (General Purpose
Register). If we want to use SDMA support for those module who do
not get the even number as default, we need to configure GPR first.

Thus this patch adds this support of GPR event remapping configuration
to the SDMA driver.

Signed-off-by: Zidan Wang <zidan.wang@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>