MIPS: SNI: Fix conflicting wrapper symbols for headers.

If Open Firmware / Device Tree support is enabled on a SNI RM kernel both
<asm/mipsprom.h> and <asm/prom.h> will be included into some .c files.
Since both headers use the same wrapper symbol only the inclusion of the
first file will have an effect but the 2nd file will be ignored resulting
in a build error.

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