Add DDR performance monitor support for iMX8QXP. The PMU consists of 3
programmable event counters and a single dedicated cycle counter.

Example usage:

 $ perf stat -a -e \
   imx8_ddr0/read-cycles/,imx8_ddr0/write-cycles/,imx8_ddr0/precharge/ ls

- or -

 $ perf stat -a -e \
   imx8_ddr0/cycles/,imx8_ddr0/read-access/,imx8_ddr0/write-access/ ls

Other events are supported, and advertised via perf list.
Reviewed-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
[will: rewrote commit message/kconfig and used #defines for dev/cpuhp names]
Signed-off-by: Will Deacon <will.deacon@arm.com>