greybus: arche-platform: Introduce FW_FLASHING state

Introduce FW_FLASHING state to arche-platform driver, to enable
user space to flash/upgrade SVC firmware.

Command to enter into flashing state:

  # echo fw_flashing > /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>