arm: mach-k3: j721e-init.c: Add support for CONFIG_K3_OPP_LOW

arm: mach-k3: j721e-init.c: Add support for CONFIG_K3_OPP_LOW

The default j7200 devicetree and k3_avs driver set 2GHz/1GHz frequency
for A72/MSMC clks and the OPP_NOM voltage.

J7200 SOCs may support OPP_LOW Operating Performance Point:
1GHz/500MHz clks for A72/MSMC and OPP_LOW AVS voltage read from efuse.

Hence, add a config check in board_init_f() to select OPP_LOW specs:
- Check if OPP_LOW AVS voltage read from efuse is valid.
- Use the device IDs and clock IDs (TISCI docs [0]) to find the A72 and
  MSMC clock frequencies in the devicetree.
- Fixup the clock frequencies in devicetree as per OPP_LOW spec.

k3_avs driver programs the OPP_LOW AVS voltage for VDD_CPU through
k3_avs_notify_freq() callback from clk_k3.

[0]: https://software-dl.ti.com/tisci/esd/latest/5_soc_doc/j7200/clocks.html

Signed-off-by: Aniket Limaye <a-limaye@ti.com>
Reviewed-by: Manorit Chawdhry <m-chawdhry@ti.com>