dmaengine: shdma: Update SH_DMAC_MAX_CHANNELS to 20

Update SH_DMAC_MAX_CHANNELS to support the 20 DMA
channels included in the sh73a0 SY-DMAC hardware.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>