of: Constify struct property pointers

Most accesses to struct property do not modify it, so constify struct
property pointers where ever possible in the DT core code.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20241010-dt-const-v1-4-87a51f558425@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>