mvebu: Add button support for GL.iNet GL-MV1000

mvebu: Add button support for GL.iNet GL-MV1000

The original patch to support this device advertised support for the reset
button and the "switch" in the commit message. But neither were actually
integrated in the device tree or documented anywere.

The button itself is now used to trigger a reset (as described in the
official GL.iNet documentation). The switch itself is registered as BTN_0
like other devices from GL.iNet in ath79.

Fixes: 050c24f05c85 ("mvebu: add support for GL.iNet GL-MV1000")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
(cherry picked from commit 01b911a9381426314762822f97e3a7e658295f50)