arm64: dts: imx8ulp: add #reset-cells for pcc

The binding file clock/imx8ulp-pcc-clock.yaml indicates '#reset-cells'
is a required property, add it.

Fixes: fe6291e9

 ("arm64: dts: imx8ulp: Add the basic dtsi file for imx8ulp")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>