arm: socfpga: soc64: Document down boot_scratch_cold register usage

arm: socfpga: soc64: Document down boot_scratch_cold register usage

Document down the usage of boot_scratch_cold register to avoid
overlapping of usage in the code for S10 & Agilex.
The boot_scratch_cold register is generally used for passing
critical system info between SPL, U-Boot and Linux.

Signed-off-by: Chin Liang See <chin.liang.see@intel.com>
Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>