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Compute the max register from the GPIO chip offset and number of GPIO
chips.

This permits to read all registers from debugfs.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: 周琰杰 (Zhou Yanjie)<zhouyanjie@wanyeetech.com>
Link: https://lore.kernel.org/r/20210717174836.14776-3-paul@crapouillou.net

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