ar71xx: add support for Aerohive AP-121

This adds support for Aerohive AP-121 access point.

Specification:
- SoC: Atheros AR9344-BC2A at 560MHz
- WiFi 1: 2.4GHz Atheros AR9340? - SoC
- WiFi 2: 5.0GHz Atheros AR9382-AL1A
- Memory: 128MB from 2x Nanya NT5TU32M16DG-AC
- SPI: 1MB Macronix MX25L8006E
- NAND: 128MB Hynix H27U1G8F2BTR-BC
- Ethernet: Atheros AR8035-A
- USB: 1x 2.0
- TPM: Atmel SC3204

Flashing:
1. Hook into UART (9600 baud) and enter U-Boot. You may need to enter
a password of administrator or AhNf?d@ta06 if prompted.
2. Once in U-Boot, download and flash LEDE factory image over tftp:

dhcp;
setenv serverip tftp-server-ip;
tftpboot 0x81000000 lede-ar71xx-nand-hiveap-121-squashfs-factory.bin;
nand erase 0x800000 0x800000;
nand write 0x81000000 0x800000 0x800000;
reset;

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>
[minor text changes in commit subject and description, fixed
alphabetical order in etc/diag.sh, use only model name in lib/ar71xx.sh,
fixed code style issues in mach-hiveap-121.c, ubinized factory image]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>