ath79, lantiq, ipq40xx, ramips all use the OpenWrt-specific gpio-export
functionality. Consolidate the patch that adds it under hack-5.10 since
this logic is obviously not target-specific. For those who want to
disable it, unsetting CONFIG_GPIO_SYSFS symbol will disable this code.
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>