ath79: add support for Buffalo BHR-4GRV

Buffalo BHR-4GRV is a wired router, based on Atheros AR7242.

Specification:

- Atheros AR7242
- 64 MB of RAM
- 32 MB of Flash
  - 2x 16 MB SPI-NOR flash
- 5x 10/100/1000 Mbps Ethernet
- 3x LEDs, 2x keys
- 1x USB 2.0 Type-A
- UART header on PCB
  - JP1: Vcc, GND, TX, RX from reset button side

Flash instruction using factory image:

1. Boot the BHR-4GRV normaly and connect the computer to its LAN port
2. Access to
"http://192.168.11.1/cgi-bin/cgi?req=frm&frm=py-db/firmup.html"
with user "bufpy" and password "otdpopy"
3. Select the OpenWrt factory image and click "OK" button to perform
firmware upgrade
4. Wait ~200 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>

target/linux/ath79/dts/ar7242_buffalo_bhr-4grv.dts

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR MIT
+/dts-v1/;
+
+#include "ar7242_buffalo_wzr-bhr.dtsi"
+
+/ {
+	compatible = "buffalo,bhr-4grv", "qca,ar7242";
+	model = "Buffalo BHR-4GRV";
+};
+
+&sec_vpn {
+	label = "buffalo:orange:vpn";
+};