ARM: shmobile: armadillo-reference: Use low level IRQ for ST1231

The device is configured to generate an active-low interrupt signal that
needs to be acknowledged by the host. Configure the IRQ to trigger on
low level.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>