libpng: Enable NEON for aarch64 to enensure consistency with arm32.

NEON is enabled for aarch64 by default, so, to ensure consistency with
arm32, reference to libpng-1.6.38/configure, added
enable_hardware_optimizations option for aarch64.

Signed-off-by: Lei Maohui <leimaohui@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>