dt-bindings: perf: marvell: cn10k ddr performance monitor

Add binding documentation for the Marvell CN10k DDR
performance monitor unit.

Signed-off-by: Bharat Bhushan <bbhushan2@marvell.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220211045346.17894-2-bbhushan2@marvell.com
Signed-off-by: Will Deacon <will@kernel.org>