mips: ath79: Add support for TPLink WDR4300

Add support for the TPLink WDR4300 router, which is based on the
AR9344 MIPS 74Kc CPU and has 128 MiB of RAM. The USB is supported
on this system as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>