ARM: dts: am43xx-clocks: add spread spectrum support

ARM: dts: am43xx-clocks: add spread spectrum support

Registers for adjusting the spread spectrum clocking (SSC) have been
added. As reported by the TI spruhl7x RM, SSC is supported only for LCD
and MPU PLLs, but the PRCM_CM_SSC_DELTAMSTEP_DPLL_XXX and
PRCM_CM_SSC_MODFREQDIV_DPLL_XXX registers, as well as the enable field
in the PRCM_CM_CLKMODE_DPLL_XXX registers are mapped for all PLLs (CORE,
MPU, DDR, PER, DISP, EXTDEV).

Link: https://lore.kernel.org/r/20210606202253.31649-5-dariobin@libero.it
Signed-off-by: Dario Binacchi <dariobin@libero.it>