sh: dma-api channel capability extensions.

This extends the SH DMA API for allowing handling of DMA
channels based off of their respective capabilities.

A couple of functions are added to the existing API,
the core bits are register_chan_caps() for registering
channel capabilities, and request_dma_bycap() for fetching
a channel dynamically based off of a capability set.

Signed-off-by: Mark Glaisher <mark.glaisher@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>