dmaengine: hdmac: Add memset capabilities

Just like for the XDMAC, the SoCs that embed the HDMAC don't have any kind
of GPU, and need to accelerate a few framebuffer-related operations through
their DMA controller.

However, unlike the XDMAC, the HDMAC doesn't have the memset capability
built-in. That can be easily emulated though, by doing a transfer with a
fixed address on the variable that holds the value we want to set.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>