riscv: dts: sifive: add missing #interrupt-cells to pmic

At W=2 dtc complains:
hifive-unmatched-a00.dts:120.10-238.4: Warning (interrupt_provider): /soc/i2c@10030000/pmic@58: Missing '#interrupt-cells' in interrupt provider

Add the missing property.
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

arch/riscv/boot/dts/sifive/hifive-unmatched-a00.dts

@@ -123,6 +123,7 @@ pmic@58 {
 		interrupt-parent = <&gpio>;
 		interrupts = <1 IRQ_TYPE_LEVEL_LOW>;
 		interrupt-controller;
+		#interrupt-cells = <2>;
 
 		onkey {
 			compatible = "dlg,da9063-onkey";