soc: ixp4xx: Add features from regmap helper

If we want to read the CFG2 register on the expansion bus and
apply the inversion and check for some hardcoded versions this
helper comes in handy.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220211223238.648934-7-linus.walleij@linaro.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>