dmaengine: rcar-dmac: Add Renesas R-Car Gen2 DMA Controller (DMAC) driver

The DMAC is a general purpose multi-channel DMA controller that supports
both slave and memcpy transfers.

The driver currently supports the DMAC found in the r8a7790 and r8a7791
SoCs. Support for compatible DMA controllers (such as the audio DMAC)
will be added later.

Feature-wise, automatic hardware handling of descriptors chains isn't
supported yet. LPAE support is implemented.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>