docs/perf: Add documentation for the i.MX8 DDR PMU

Add some documentation describing the DDR PMU residing in the Freescale
i.MDX SoC and its perf driver implementation in Linux.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Will Deacon <will@kernel.org>