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>