clk: qcom: rpmh: skip undefined clocks when registering

When iterating over a platform's available clocks in clk_rpmh_probe(),
check for undefined (null) entries in the clocks array.  Not all
clock indexes necessarily have clocks defined.

Signed-off-by: Taniya Das <tdas@codeaurora.org>
Link: https://lkml.kernel.org/r/1578305923-29125-2-git-send-email-tdas@codeaurora.org
[sboyd@kernel.org: Leave 'name' declaration at beginning of loop]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>