greybus: firmware: Add firmware-download protocol driver

This patch adds Firmware Download Protocol support to firmware core,
which allows an Interface to download a firmware package over Unipro.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Jun Li <li_jun@projectara.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>