MIPS: ath25: add Wireless device support

Atheros AR5312 and AR2315 both have a builtin wireless device, this
patch add helper code and register platform device for all supported
WiSoCs.

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/8249/


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