greybus: arche_platform: Add sysfs to allow user to change state

This patch introduces sysfs interface for the user space to enable
operational state change of the driver.
Currently, driver supports, 'off', 'active' and 'standby'

Note that, driver doesn't do anything for standby state as of today.

To see the current state

      # cat /sys/devices/arche_platform.*/state

And to change the state
      # echo [off/active/standby] > /sys/devices/arche_platform.*/state

Testing Done: Tested on EVT1.2 and DB3.5 platform.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>