ipq806x: Add support for Netgear Nighthawk Pro Gaming XR450

ipq806x: Add support for Netgear Nighthawk Pro Gaming XR450

Motivation:
 Firmware with platform ID "XR450" required for easy installation.

Manufacturer pages:
 Product page: https://www.netgear.com/support/product/xr450
 Product Data Sheet: https://www.netgear.com/images/datasheet/networking/wifirouter/XR450.pdf
 Current firmware: https://www.downloads.netgear.com/files/GDC/XR450/XR450-V2.3.2.134.zip

Specifications:
Almost identical to Netgear XR500(https://openwrt.org/toh/hwdata/netgear/netgear_xr500)

Differences:
	Platform ID: XR450, utilized for firmware validation
	WLAN 2.4GHz: b/g/n Supports QAM64 (XR500 QAM256)

SoC: Qualcomm Atheros IPQ8065
RAM: 512 MB
Storage: 256 MiB
Wireless: Qualcomm Atheros QCA9984
Switch: Qualcomm Atheros AR8337 rev. 2
Ethernet: 5x 1000/100/10
USB: 2x 3.0

Install via WEB GUI (recommended):
1. Navigate within GUI to firmware update section
2. Select openwrt-ipq806x-netgear_xr450-squashfs-factory.img
3. Dismiss downgrade warning

Install via TFTP recovery (same as XR500/R7800):

1. Configure host PC to use static address 192.168.1.10 https://openwrt.org/toh/netgear/r7800#prerequisites_for_tftp_flashing
2. Flash openwrt-ipq806x-netgear_xr450-squashfs-factory.img using procedure https://openwrt.org/toh/netgear/r7800#tftp_flashing_process

Revert to stock firmware
1. Download current manufacturer firmware, decompress zip file to obtain img file
2. Use TFTP recovery method flash img file (currently XR450-V2.3.2.134.img)
ipq806x: add support to netgear_xr450

Signed-off-by: Andrzej Mialkowski <andrzej.mialkowski@gmail.com>