dt-bindings: fix warnings in validation of qcom,gcc.yaml

The last example in qcom,gcc.yaml set 'sleep' as the second
value of 'clock-names'. According to the schema is should
be 'sleep_clk'. Fix the example to conform the schema.
This fixes a warning when validating the schema:
"clock-names:  ... is not valid under any of the given schemas"

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Link: https://lkml.kernel.org/r/20200122135741.12123-1-dafna.hirschfeld@collabora.com


Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>