dma: ti: k3-udma: Fix BCDMA probe by adding check for MMR_RFLOW

RFLOW config related MMR does not exist incase of BCDMA.
Add check to bypass the RFLOW MMR extraction.
Without this, the probe sequence fails checking for
the MMR_RFLOW region, which is valid only for packet based
DMA and obselete for BCDMA.

Fixes: 5abb694d ("dma: ti: k3-udma: Add support for native configuration of chan/flow")
Signed-off-by: Prasanth Babu Mantena <p-mantena@ti.com>
Tested-by: Jonathan Humphreys <j-humphreys@ti.com>