dts: msm8996: import PMIC dtsi files

Import PM8994 and PMI8994 DTSI files in preparation for switching
MSM8996 boards to upstream DTS.

Taken from kernel tag v6.7

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>