subsys/mgmt/ec_host_cmd: rename peripheral to backend

Follow naming pattern in the subsystems(logging or shell) and name
the layer between generic handler and peripheral driver "backend".

The name doesn't suit that well to the SHI backend, because there isn't
SHI API itself and the SHI interface is used only for the host
communication. So the backend code includes the peripheral driver itself.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>