subsys/mgmt/ec_host_cmd: update directory structure

The Host Commands can be used with different transport layers e.g. SHI
or eSPI. The code that provides the peripheral API and allows sending
and receiving Host Commands via different transport layers is not
actually drivers of a peripheral, so move it to the
subsys/mgmt/ec_host_cmd folder.

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