dt-binding: perf: Add Amlogic DDR PMU

Add binding documentation for the Amlogic G12 series DDR
performance monitor unit.

Signed-off-by: Jiucheng Xu <jiucheng.xu@amlogic.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221121021602.3306998-3-jiucheng.xu@amlogic.com


Signed-off-by: Will Deacon <will@kernel.org>