of/address: Add of_property_read_reg() helper

Add a helper, of_property_read_reg(), to read "reg" entries untranslated
address and size. This function is intended mainly for cases with an
untranslatable "reg" address (i.e. not MMIO). There's also a few
translatable cases such as address cells containing a bus chip-select
number.

Link: https://lore.kernel.org/r/20230328-dt-address-helpers-v1-5-e2456c3e77ab@kernel.org


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