MIPS: ath79: Add early printk support for the AR933X SoCs

The AR933X SoCs are using a different UART, thus require
different code for early printk support.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Kathy Giori <kgiori@qca.qualcomm.com>
Cc: "Luis R.  Rodriguez" <rodrigue@qca.qualcomm.com>
Patchwork: https://patchwork.linux-mips.org/patch/2521/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>