MIPS: fw: arc: use call_o32 to call ARC prom from 64bit kernel

When using a 64bit kernel with generic spaces setup stack is
also placed in XKPYHS, which the 32bit PROM can't handle.
By using call_o32 for ARC_CALLs a stack placed in KSEG0 is used
when calling PROM.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org