greybus: interface: add power_state attribute

User space needs the capability of powering ON or OFF an Interface for
multiple use cases. For example, userspace may want an Interface
currently in its S3 boot stage, to boot into its S2 Loader stage to
update the bridge SPI flash. Or the Interface is running its S2 Loader
stage and updated the SPI flash with the new S2 Loader firmware and
wants to boot into the new S2 Loader firmware.

Another use case can be, Android wants to disable (not eject) a
misbehaving module.

Add a 'power_state' sysfs file within the interface directory. It can be
read to know the current power state of the Interface and can be written
to power ON or power OFF an Interface. Possible values that can be
written or read from it are: "on" and "off".

Testing Done: Tested by enabling/disabling camera module on EVT 2.0.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
CC: David Lin <dtwlin@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
[johan: drop es3-quirk duplication, add to power attribute group, fix
return value, drop tags ]
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Sandeep Patil <sspatil@google.com>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>