arm64: dts: allwinner: h6: Fix PMU compatible

The commit 7aa9b9eb ("arm64: dts: allwinner: H6: Add PMU mode")
introduced support for the PMU found on the Allwinner H6. However, the
binding only allows for a single compatible, while the patch was adding
two.

Make sure we follow the binding.

Fixes: 7aa9b9eb ("arm64: dts: allwinner: H6: Add PMU mode")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>