clk: stm32f4: support spread spectrum clock generation

Support spread spectrum clock generation for the main PLL, the only one
for which this functionality is available.

Tested on the STM32F469I-DISCO board.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>

Link: https://lore.kernel.org/r/20250114182021.670435-5-dario.binacchi@amarulasolutions.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>