msm: rpm-regulator-smd: Correct qcom,system-load property name

Change of_property_read_u32() call so that it uses
"qcom,system-load" instead of "qcom,system_load".  The former
value is what shows up in device tree binding documentation.

Change-Id: I5141335e4065df54f217d12f7dd7a868bac5b279
Signed-off-by: David Collins <collinsd@codeaurora.org>