MIPS: ath25: add common parts

Add common code for Atheros AR5312 and Atheros AR2315 SoCs families.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Linux MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/8237


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