ipq40xx: add support for ASUS RT-ACRH17/RT-AC42U

SOC:	IPQ4019
CPU:	Quad-core ARMv7 Processor [410fc075] revision 5 (ARMv7), cr=10c5387d
DRAM:	256 MB
NAND:	128 MiB Macronix MX30LF1G18AC
ETH:	Qualcomm Atheros QCA8075 Gigabit Switch (4x LAN, 1x WAN)
USB:	1x 3.0 (via Synopsys DesignWare DWC3 controller in the SoC)
WLAN1:	Qualcomm Atheros QCA4019 2.4GHz 802.11bgn 2x2:2
WLAN2:	Qualcomm Atheros QCA9984 5GHz 802.11nac 4x4:4
INPUT:	1x WPS, 1x Reset
LEDS:	Status, WIFI1, WIFI2, WAN (red & blue), 4x LAN

This board is very similar to the RT-ACRH13/RT-AC58U. It must be flashed
with an intermediary initramfs image, the jffs2 ubi volume deleted, and
then finally a sysupgrade with the final image performed.

Signed-off-by: Joshua Roys <roysjosh@gmail.com>
(added ALT0)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>