MIPS: ath25: Add basic AR2315 SoC support

Add basic support for Atheros AR2315+ SoCs: registers definition file
and initial setup code.

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/8239/


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