arm64: dts: hisilicon: Standardize Poplar GPIO line names

The hi6220-HiKey board started to name GPIO lines for
96boards, using just the plain names "GPIO-A" etc from the
96boards specification.

Poplar started to use an arbitrary "LS-GPIO-A" (etc) prefix
that is not part of the 96boards specification.

As the former notation arrived first, and we need
consistency among 96board, rectify the Poplar board to use
this too. This is important for userspace that wants to
look up GPIO names from these strings.

Cc: Jiancheng Xue <xuejiancheng@hisilicon.com>
Cc: Alex Elder <elder@linaro.org>
Cc: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>