[ARM] 4769/2: ixp4xx: Button updates for the dsmg600 board

 * Remove the superfluous declaration of ctrl_alt_del().
 * Convert GPIO and IRQ handling to use the <asm/gpio.h> api.
 * Perform the reset on the release of the power button, so that
   NAS devices which have been set to auto-power-on (by solder
   bridging the power button) do not continuously power cycle.
 * Remove all superflous constants from dsmg600.h

Signed-off-by: Rod Whitby <rod@whitby.id.au>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>