PL (programmable logic) uses FCLK0..FCLK3 as a clock sources.
Normally they're configured by first stage boot loader (FSBL)
and normal user never has to touch them. These sysctls may come
useful for hardware developers
hw.fpga.fclk.N.source: clock source (IO, DDR, ARM)
hw.fpga.fclk.N.freq: requested frequency in Hz
hw.fpga.fclk.N.actual_freq: actual frequency in Hz (R/O)
hw.fgpa.level_shifters: 0/1 to enable/disable PS-PL level shifters,
normally they're enabled either by FSBL or after programming
FPGA through devcfg(4)
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