clk: ti: am33xx: add DPLL clock drivers

The digital phase-locked loop (DPLL) provides all interface clocks and
functional clocks to the processor of the AM33xx device. The AM33xx
device integrates five different DPLLs:
 * Core DPLL
 * Per DPLL
 * LCD DPLL
 * DDR DPLL
 * MPU DPLL

The patch adds support for the compatible strings:
 * "ti,am3-dpll-core-clock"
 * "ti,am3-dpll-no-gate-clock"
 * "ti,am3-dpll-no-gate-j-type-clock"
 * "ti,am3-dpll-x2-clock"

The code is loosely based on the drivers/clk/ti/dpll.c drivers of the
Linux kernel version 5.9-rc7.
For DT binding details see:
- Documentation/devicetree/bindings/clock/ti/dpll.txt

Signed-off-by: Dario Binacchi <dariobin@libero.it>