ar71xx: Add kernel 4.9 support

This add support for kernel 4.9 to the ar71xx target.
It was compile tested with the generic, NAND and mikrotik subtarget.
Multiple members of the community tested it on their boards and did not
report any major problem so far.

Especially the NAND part received some changes to adapt to the new
kernel APIs. The serial driver hack used for the Arduino Yun was not
ported because the kernel changed there a lot.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ar71xx/patches-4.9/452-gpio-add-gpio-latch-driver.patch

@@ -0,0 +1,22 @@
+--- a/drivers/gpio/Kconfig
++++ b/drivers/gpio/Kconfig
+@@ -1218,4 +1218,9 @@ config GPIO_NXP_74HC153
+ 	  Platform driver for NXP 74HC153 Dual 4-input Multiplexer. This
+ 	  provides a GPIO interface supporting input mode only.
+ 
++config GPIO_LATCH
++	tristate "GPIO latch driver"
++	help
++	  Say yes here to enable a GPIO latch driver.
++
+ endif
+--- a/drivers/gpio/Makefile
++++ b/drivers/gpio/Makefile
+@@ -56,6 +56,7 @@ obj-$(CONFIG_GPIO_JANZ_TTL)	+= gpio-janz
+ obj-$(CONFIG_GPIO_KEMPLD)	+= gpio-kempld.o
+ obj-$(CONFIG_ARCH_KS8695)	+= gpio-ks8695.o
+ obj-$(CONFIG_GPIO_INTEL_MID)	+= gpio-intel-mid.o
++obj-$(CONFIG_GPIO_LATCH)	+= gpio-latch.o
+ obj-$(CONFIG_GPIO_LOONGSON)	+= gpio-loongson.o
+ obj-$(CONFIG_GPIO_LP3943)	+= gpio-lp3943.o
+ obj-$(CONFIG_GPIO_LPC18XX)	+= gpio-lpc18xx.o