riscv: dts: microchip: re-add pdma to mpfs device tree

PolarFire SoC /does/ have a SiFive pdma, despite what I suggested as a
conflict resolution to Zong. Somehow the entry fell through the cracks
between versions of my dt patches, so re-add it with Zong's updated
compatible & dma-channels property.

Fixes: c5094f37

 ("riscv: dts: microchip: refactor icicle kit device tree")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>