MIPS: sgi-ip27: Fix missing prototypes

Fix missing prototypes by making not shared functions static and
adding others to ip27-common.h. Also drop ip27-hubio.c as it's
not used for a long time.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>