watchdog: imgpdc: Drop METAG dependency

Now that arch/metag/ has been removed, remove the METAG dependency from
the IMG IR device driver. The hardware is also present on MIPS SoCs so
the driver still has value.

Signed-off-by: James Hogan <jhogan@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-metag@vger.kernel.org