'reg' is the standard property for defining register banks/addresses. Add
it to use for the register address and deprecate 'offset'. This also
allows for using standard node names with unit-addresses. However, since
it is quite possible to have multiple nodes at the same register
address, allow for the unit-address to optionally have the bit
offset. The unit-address format is '@<reg address>[,<bit offset>]'. This
matches the format recently added for nvmem binding which has the same
issue.

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-leds@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210913192816.1225025-3-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>