qcom: msm-pm: Do not fail on L2 clk get failure

On some targets the l2 clock does not have a separate reference. The clock
is enabled/disabled based on the cores cpu clock references.
Do not fail probe if clock get on l2 clock fails.

Change-Id: I7949fac5789acb7aa1acff5eb242ee83288bf044
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>