dt-bindings: mfd: Convert pm8xxx bindings to yaml

Convert pm8xxx bindings from .txt to .yaml format. Also,
split this binding into two: parent binding(qcom-pm8xxx.yaml)
and child node RTC binding(qcom-pm8xxx-rtc.yaml).

Signed-off-by: satya priya <skakit@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>