greybus: svc: implement svc_intf_{vsys,refclk,unipro}_{enable,disable} operations

Add SVC operations for fine grain control over vsys, refclk, and unipro
port power.

Testing done: used "new" firmware boot sequence to verify that modules
              were correctly detected and booted.

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>