[POWERPC] cell: handle kernel SLB setup in spu_base.c

Currently, the SPU context switch code (spufs/switch.c) sets up the
SPU's SLBs directly, which requires some low-level mm stuff.

This change moves the kernel SLB setup to spu_base.c, by exposing
a function spu_setup_kernel_slbs() to do this setup. This allows us
to remove the low-level mm code from switch.c, making it possible
to later move switch.c to the spufs module.

Also, add a struct spu_slb for the cases where we need to deal with
SLB entries.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>