arm64: rockchip: Don't always put PLL to normal mode

We used to put every PLL in normal mode (meaning that the output would
be the result of the PLL configuration) instead of slow mode (the output
is equal to the external oscillator frequency, 24-26Mhz) but this doesn't
work for most of the PLLs as when we put them into normal mode the registers
configuring the output frequency haven't been set.
Add a normal_mode member in clk_pll_def/clk_pll_sc struct and if it's true
we then set the PLL to normal mode.
For now only set it to the LPLL and BPLL (Little cluster PLL and Big cluster
PLL respectively).

Reviewed by: ganbold
Differential Revision: https://reviews.freebsd.org/D20174