arm64: dts: hikey: name the GPIO lines

This names the GPIO lines on the HiKey board in accordance with
the 96Board Specification for especially the Low Speed External
Connector: "GPIO-A" thru "GPIO-L".

This will make these line names reflect through to userspace
so that they can easily be identified and used with the new
character device ABI.

Some care has been taken to name all lines, not just those used
by the external connectors, also lines that are muxed into some
other function than GPIO: these are named "[FOO]" so that users
can see with lsgpio what all lines are used for.

Cc: devicetree@vger.kernel.org
Cc: John Stultz <john.stultz@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: David Mandala <david.mandala@linaro.org>
Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Wei Xu <xuwei5@hisilicon.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>