Make it clear that ACPI needs to be present only to get driver functional.
It is not required for compilation.

Fixes: 356b01a9 ("gpio: gpio-hisi: Add HiSilicon GPIO support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20201214165524.43843-1-andriy.shevchenko@linux.intel.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>