powerpc: Export cpu_to_chip_id() to fix build error

powerpc allmodconfig build fails with:

ERROR: ".cpu_to_chip_id" [drivers/block/mtip32xx/mtip32xx.ko] undefined!

The problem was introduced with commit 15863ff3 (powerpc: Make chip-id
information available to userspace).

Export the missing symbol.

Cc: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Cc: Shivaprasad G Bhat <sbhat@linux.vnet.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>