ath79: add support for PQI Air Pen

ath79: add support for PQI Air Pen

SoC: AR9330 (or AR9331 revision?)
Ethernet x1, Wireless 2.4G, uSD card slot x1.
USB Power, include Li-Po Battery.

Flash 8MiB. (factory is dual image)
1. write image is use half 4MiB size from factory.
  write initramfs image to mtd6 and mtd7.
2. change next boot setting. next is mtd6 boot.
  echo -ne "\x01" | dd of=/dev/mtdblock8 conv=notrunc bs=1 count=1 seek=0

3. rewrite sysupgrade image.
  change next boot setting. next is mtd4.
  echo -ne "\x00" | dd of=/dev/mtdblock8 conv=notrunc bs=1 count=1 seek=0

OEM mtd.
dev:    size   erasesize  name
mtd0: 00040000 00010000 "u-boot"
mtd1: 00010000 00010000 "u-boot-env"
mtd2: 00010000 00010000 "ART"
mtd3: 00010000 00010000 "NVRAM"
mtd4: 00140000 00010000 "uImage"
mtd5: 00280000 00010000 "rootfs"
mtd6: 00140000 00010000 "uImage2"
mtd7: 00280000 00010000 "rootfs2"
mtd8: 00010000 00010000 "CONF"

Signed-off-by: YuheiOKAWA <tochiro.srchack@gmail.com>