MIPS: ath79: add early_printk support for AR934X

The patch allows to see kernel messages on AR934X SoCs in
early boot stage.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Cc: linux-mips@linux-mips.org
Cc: mcgrof@infradead.org
Patchwork: https://patchwork.linux-mips.org/patch/3504/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>