dt-bindings: atmel-sysreg: add microchip,lpm-connection binding

Add microchip,lpm-connection binding which allows to specify the devices
the SHDWC's LPM pin is connected.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>

Author: claudiubeznea

Documentation/devicetree/bindings/arm/atmel-sysregs.txt

@@ -119,6 +119,10 @@ optional properties:
 optional microchip,sam9x60-shdwc or microchip,sama7g5-shdwc properties:
 - atmel,wakeup-rtt-timer: boolean to enable Real-time Timer Wake-up.
 
+optional microchip,sama7g5-shdwc properties:
+- microchip,lpm-connection: list of phandles to devices which are connected
+  to SHDWC's LPM pin
+
 The node contains child nodes for each wake-up input that the platform uses.
 
 2) input nodes