powerpc: Create a helper for getting the kernel toc value

Move the logic to work out the kernel toc pointer into a header. This is
a good cleanup, and also means we can use it elsewhere in future.

Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>