ath79: add support for D-Link DIR-842 C2

ath79: add support for D-Link DIR-842 C2

Hardware spec of DIR-842 C2:
SoC: QCA9563
DRAM: 128MB DDR2
Flash: 16MB SPI-NOR
Switch: QCA8337N
WiFi 5.8GHz: QCA9888
WiFi 2.4Ghz: QCA9563
USB: 2.0

Flash instructions:

1. Upgrade the factory.bin through the factory web interface or
   the u-boot failsafe interface.
   The firmware will boot up correctly for the first time.
   Do not power off the device after OpenWrt has booted.
   Otherwise the u-boot will enter failsafe mode as the checksum
   of the firmware has been changed.
2. Upgrade the sysupgrade.bin in OpenWrt.
   After upgrading completes the u-boot won't complain about the
   firmware checksum and it's OK to use now.
3. If you powered off the device before upgrading the sysupgrade.bin,
   just upgrade the factory.bin through the u-boot failsafe interface
   and then goto step 2.

Signed-off-by: Jackson Lim <jackcolentern@gmail.com>
[Reword reset-hog comment, fix formatting]
Signed-off-by: David Bauer <mail@david-bauer.net>