ARM: at91: pm: use of_device_id array to find the proper shdwc node

Use of_device_id array to find the proper shdwc compatibile node.
SAM9X60's shdwc changes were not integrated when
commit eaedc0d3 ("ARM: at91: pm: add ULP1 support for SAM9X60")
was integrated.

Fixes: eaedc0d3

 ("ARM: at91: pm: add ULP1 support for SAM9X60")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1576062248-18514-3-git-send-email-claudiu.beznea@microchip.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>