docs/perf: Add documentation for the Amlogic G12 DDR PMU

Add a user guide to show how to use DDR PMU to
monitor DDR bandwidth on Amlogic G12 SoC

Signed-off-by: Jiucheng Xu <jiucheng.xu@amlogic.com>
Reviewed-by: Chris Healy <healych@amazon.com>
Link: https://lore.kernel.org/r/20221121021602.3306998-2-jiucheng.xu@amlogic.com


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