ipq40xx: add support for Netgear RBR20

ipq40xx: add support for Netgear RBR20

This device shares a lot of similarities with the LBR20 - the RBR20 just
misses the LTE modem and its formfactor is alot smaller. Other than that
the LED configuration matches other RBR devices but the RBR20 has less
LEDs than its larger counterparts.

Hardware
--------
SOC:    Qualcomm IPQ4019
FLASH:	256MB (Winbond W29N02GVSIAF)
RAM:  	512MB (Nanya NT5CC256M16EP-EK)
WIFI:   Qualcomm IPQ4019
        Qualcomm Atheros QCA9886
ETH:	1x WAN, 1x LAN
LED:	5 (4 RGB at top, 1 RG at back)
BTN:	WPS, Reset
UART:	115200 8N1 (dotted Pin = VCC) VCC-TX-RX-GND

MAC addresses
-------------
LAN	Label MAC (stored in boarddata1 offset 0x0)
WAN	LAN + 1 (stored in boarddata1 offset 0x6)
2.4G	LAN
5GLow	LAN + 3 (stored in boarddata1 offset 0xc)
5GUpper LAN + 2 (stored in boarddata1 offset 0x12)

Installation
------------
Either use the vendor ui upgrade method or nmrpflash to install the
factory image.

Signed-off-by: Christoph Krapp <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20560
Signed-off-by: Robert Marko <[email protected]>