ARM: at91/dt: keep watchdog running in idle mode

Since turning on idle-halt in commit fe46aa679f12 (ARM: at91/dt: add
sam9 watchdog default options to SoCs), SoCs compatible with at91sam9260-wdt
no longer reboot if the watchdog times out while the CPU is in idle state.
Removing the 'idle-halt' flag that was set by default fixes this.

Signed-off-by: Michel Marti <mma@objectxp.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
[nicolas.ferre@atmel.com: rework the commit message]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>