ARM: dts: imx6sll: remove unused property in gpc node

The "fsl,mf-mix-wakeup-irq" is ONLY used as a temporary
solution in NXP's internal tree for Mega/Fast Mix off
feature after suspend, upstream kernel does NOT need it,
remove it.

Signed-off-by: Anson Huang <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6sll.dtsi

···
 				#interrupt-cells = <3>;
 				interrupts = <GIC_SPI 89 IRQ_TYPE_LEVEL_HIGH>;
 				interrupt-parent = <&intc>;
-				fsl,mf-mix-wakeup-irq = <0x7c00000 0x7d00 0x0 0x1400640>;
 			};
 
 			iomuxc: pinctrl@20e0000 {