PM / devfreq: rockchip-dfi: Add perf support

The DFI is a unit which is suitable for measuring DDR utilization, but
so far it could only be used as an event driver for the DDR frequency
scaling driver. This adds perf support to the DFI driver.

Usage with the 'perf' tool can look like:

perf stat -a -e rockchip_ddr/cycles/,\
		rockchip_ddr/read-bytes/,\
		rockchip_ddr/write-bytes/,\
		rockchip_ddr/bytes/ sleep 1

 Performance counter stats for 'system wide':

        1582524826      rockchip_ddr/cycles/
           1802.25 MB   rockchip_ddr/read-bytes/
           1793.72 MB   rockchip_ddr/write-bytes/
           3595.90 MB   rockchip_ddr/bytes/

       1.014369709 seconds time elapsed

perf support has been tested on a RK3568 and a RK3399, the latter with
dual channel DDR.

Link: https://lore.kernel.org/all/[email protected]/
Reviewed-by: Sebastian Reichel <[email protected]>
Acked-by: Chanwoo Choi <[email protected]>
Acked-by: Heiko Stuebner <[email protected]>
Signed-off-by: Sascha Hauer <[email protected]>
[cw00.choi: Fix typo from 'write_acccess' to 'write_access']
Signed-off-by: Chanwoo Choi <[email protected]>

Author: saschahauer