dt-bindings: clock: add ipq6018 a53 pll compatible

cpus on ipq6018 are clocked by a53 pll, add device compatible for a53
pll found on ipq6018 devices.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sivaprakash Murugesan <sivaprak@codeaurora.org>
Link: https://lore.kernel.org/r/1592800092-20533-2-git-send-email-sivaprak@codeaurora.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>