gpio: acpi: separation of concerns

The generic GPIO library directly implement code for acpi_find_gpio()
which is only used with CONFIG_ACPI. This was probably done because
OF did the same thing, but I removed that so remove this too.

Rename the internal acpi_find_gpio() in gpiolib-acpi.c to
acpi_populate_gpio_lookup() which seems to be more appropriate anyway
so as to avoid a namespace clash with the same function.

Make the stub return -ENOENT rather than -ENOSYS (as that is for
syscalls!).

For some reason the sunxi pin control driver was including the private
gpiolib header, it works just fine without it so remove that oneliner.

Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>