ec_host_cmd_periph: add device API

The host command peripheral device API abstracts how an embedded
controller sends and receives data from a host on a bus. Each bus like
eSPI, SPI, or I2C would implement their own host command peripheral
device. Each hardware device would then handle the necessary hardware
access to send and receive data over that bus.

The chosen host command peripheral device will be used by the host
command handler framework to send and receive host data correctly.

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