powerpc/booke: Correct the SPRN_MAS5 definition.

339 is the SPR number for MAS5 documented by Power ISA 2.06, and
implemented by e500mc.  It is not yet used anywhere in the kernel,
so nothing should be relying on the wrong number.

Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>