MLK-21289 perf: ddr-perf: calculate ddr bandwidth via virtual event read-bytes/write...

MLK-21289 perf: ddr-perf: calculate ddr bandwidth via virtual event read-bytes/write-bytes

We can calculate ddr bandwidth via virtual event read-bytes/write-bytes based
on ddr burst width, which actually share event read-cycles/write-cycles. Burst
width is 32bit on i.MX8 board till now.

The ddr interface will generate 2 up edges and 2 down edges in an internal
clock cycle, so it can pass 4 beats of data. 4 bytes of each beat if ddr
burst width is 32 bit.

Cmd bellow:
perf stat -a -e ddr0/read-bytes/ ls
perf stat -a -e ddr0/write-bytes/ ls

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>