greybus: svc: implement svc_intf_activate

With upcoming firmware changes we will switch from an SVC-driven module
boot sequence to an AP-driven module sequence. This operation allows the
AP to request the SVC to boot a module to which the AP has previouslt
requested power be applied. This operation will also determine if the
remote interface is a dummy module, UniPro-only module, or full Greybus
module.

Testing done: Tested together with "new" firmware boot sequence to
              verify that modules are detected and booted as expected.

Signed-off-by: Jeffrey Carlyle <jcarlyle@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>