ar71xx: add TP-Link TL-WA901ND-v4 support

target/linux/ar71xx/base-files/etc/board.d/01_leds

target/linux/ar71xx/base-files/etc/board.d/02_network

target/linux/ar71xx/base-files/etc/diag.sh

target/linux/ar71xx/base-files/lib/ar71xx.sh

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

target/linux/ar71xx/config-4.1

target/linux/ar71xx/files/arch/mips/ath79/Kconfig.openwrt

target/linux/ar71xx/files/arch/mips/ath79/Makefile

...t/linux/ar71xx/files/arch/mips/ath79/mach-tl-wa901nd-v4.c

target/linux/ar71xx/files/arch/mips/ath79/machtypes.h

target/linux/ar71xx/image/Makefile

tools/firmware-utils/src/mktplinkfw.c