dmaengine: ti-dma-crossbar: Fix event mapping for TPCC_EVT_MUX_60_63

Register layout of a typical TPCC_EVT_MUX_M_N register is such that the
lowest numbered event is at the lowest byte address and highest numbered
event at highest byte address. But TPCC_EVT_MUX_60_63 register layout is
different,  in that the lowest numbered event is at the highest address
and highest numbered event is at the lowest address. Therefore, modify
ti_am335x_xbar_write() to handle TPCC_EVT_MUX_60_63 register
accordingly.

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