freedreno: Add A2xx REG_A2XX_RBBM_PM_OVERRIDE2 bitfields

Dmitry pointed me to

https://github.com/genesi/linux-legacy/blob/master/drivers/mxc/amd-gpu/include/reg/yamato/10/yamato_mask.h#L4428

this has even more nice reg descriptions, might be worth looking into some day

required for https://lore.kernel.org/linux-arm-msm/20230223-topic-opp-v2-0-24ed24cd7358@linaro.org/ to rid magic values from a2xx_busy()