arch: nios2: remove 'resetvalue' property

The 'altr,pio-1.0' driver does not handle the 'resetvalue', so remove it.

Signed-off-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Ley Foon Tan <[e-mail]>

Documentation/devicetree/bindings/fpga/fpga-region.txt

···
 				reg = <0x10040 0x20>;
 				clocks = <0x2>;
 				altr,ngpio = <0x4>;
-				resetvalue = <0x0>;
 				#gpio-cells = <0x2>;
 				gpio-controller;
 			};

arch/nios2/boot/dts/10m50_devboard.dts

···
 			compatible = "altr,pio-1.0";
 			reg = <0x180014d0 0x00000010>;
 			altr,ngpio = <4>;
-			resetvalue = <15>;
 			#gpio-cells = <2>;
 			gpio-controller;
 		};
···
 			altr,interrupt-type = <2>;
 			edge_type = <1>;
 			level_trigger = <0>;
-			resetvalue = <0>;
 			#gpio-cells = <2>;
 			gpio-controller;
 		};