greybus: arche-apb-ctrl: Add sysfs to allow user to change state

This patch introduces sysfs interface for the user space to enable
state change of the driver. Driver supports below operational
states,

 - off
 - active
 - standby
 - fw_flashing

To see the current state

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

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