MIPS: Update arch/mips/include/asm/sgi/sgi.h

Update arch/mips/include/asm/sgi/sgi.h with some updated information on SGI
systems.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Cc: Linux MIPS List <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/8666/


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