MIPS: ath79: Use the IRQ based GPIO key driver for the buttons

Now that the GPIO driver support interrupts we don't need to poll the
buttons.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/15283/
Cc: linux-mips@linux-mips.org
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org