arm64: allwinner: Add 792Mhz frequency to sun50i-a64-opp

This is the frequency of the cpu on the Pinebook so add it to make
cpufreq find the current setting.
Note that this dtbo on the Pinebook doesn't work right now as u-boot
dtb doesn't have symbols and so it fails to apply. Linux 4.20 have
the dts and will be imported once taggued.

MFC after: 1 month
X-MFC with: r341268