dt-bindings: mhi: add support to extract chtype from devicetree

Depending on MHI channel, chtype bits can be used to describe
additional properties apart from channel direction. Updating
devicetree documentation to describe such properties.

CRs-Fixed: 2361266
Change-Id: Icffe80b5e3af8d19418ed21f7bc02613db13f376
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>