ARM: dts: stm32: Fix and expand PLL configuration comments

ARM: dts: stm32: Fix and expand PLL configuration comments

Fix the frequencies listed in PLL configuration comments to match
the actual frequencies programmed into hardware. Furthermore, add
a comment which explains how those frequencies are calculated, so
it won't be necessary to look it up all over the datasheet and
make more mistakes in the calculation in the future.

Signed-off-by: Marek Vasut <[email protected]>
Cc: Patrice Chotard <[email protected]>
Cc: Patrick Delaunay <[email protected]>
Reviewed-by: Patrick Delaunay <[email protected]>