perf/imx_ddr: Add support for PMU in DB (system interconnects)

perf/imx_ddr: Add support for PMU in DB (system interconnects)

There is a PMU in DB, which has the same function with PMU in DDR
subsystem, the difference is PMU in DB only supports cycles, axid-read,
axid-write events.

e.g.
perf stat -a -e imx8_db0/axid-read,axi_mask=0xMMMM,axi_id=0xDDDD,axi_port=0xPP,axi_channel=0xH/ cmd
perf stat -a -e imx8_db0/axid-write,axi_mask=0xMMMM,axi_id=0xDDDD,axi_port=0xPP,axi_channel=0xH/ cmd

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