greybus: arche-platform: Pull wake/detect low in poweroff

When SVC goes down, make sure that wake/detect is also pulled low.
Note that this is not the criteria for SVC poweroff, but it is required
to have right state on wake/detect line before powering off.
And on next boot, the boot-sequence code would take care of
handshaking with SVC.

Tested-on: 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>