The original code of gpiodev_add_to_list is not very clear which
lead to bugs or compiling warning, reference the following patches:
Bugs:
1.  Commit ef7c7553 ("gpiolib: improve overlap check of range of
    gpio").
2.  Commit 96098df1 ("gpiolib: fix chip order in gpio list")

Warning:
1.  Commit e28ecca6 ("gpio: fix warning about iterator").
of gpio").

There is a off-list discussion about how to improve it consequently.
This commit try to follow this by rewriting the whole functions.

Tested pass with my gpio mockup driver and test scripts[1].

[1] http://www.spinics.net/lists/linux-gpio/msg09598.htmlSuggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>