bus: mhi: ep: Add support for processing command ring

Add support for processing the command ring. Command ring is used by the
host to issue channel specific commands to the ep device. Following
commands are supported:

1. Start channel
2. Stop channel
3. Reset channel

Once the device receives the command doorbell interrupt from host, it
executes the command and generates a command completion event to the
host in the primary event ring.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>