ec_host_cmd: add ec host command handler framework

Add a generic host command handler framework that allows users to
declare new host command handlers with the HOST_COMMAND_HANDLER macro
at build time. The framework will handle incoming messages from the
host command peripheral device and forwards the incoming data to the
appropriate host command handler, which is looked up by id.

The framework will also send the response from the handler back to the
host command peripheral device. The device handles sending the data on
the physical bus.

This type of host command communication is typically done on an embedded
controller for a notebook or computer. The host would be the main
application processor (aka AP, CPU, SoC).

Signed-off-by: Jett Rink <jettrink@google.com>