dt-bindings: mfd: Add compatible for pmk8350 rtc

Add compatible string for pmk8350 rtc support.

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