arm64: dts: qcom: sa8540p-pmics: rename pmic labels

The SA8540P PMICs are named PMM8540. Rename the devicetree source labels
to reflect this.

Reviewed-by: Brian Masney <bmasney@redhat.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Eric Chanudet <echanude@redhat.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230111160335.7175-3-johan+linaro@kernel.org