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

[ Upstream commit ce6b6d15 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>