clk: tegra: bpmp: Don't crash when a clock fails to register

When registering clocks, we just skip any that fail to register
(leaving a NULL hole in the clock table). However, our of_xlate
function still tries to dereference each entry while looking for
the clock with the requested id, causing a crash if any clocks
failed to register. Add a check to of_xlate to skip any NULL
clocks.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>