powerpc/8xx: Declare SPRG2 as a SCRATCH register

Since commit 469d62be9263b92f2c3329540cbb1c076111f4f3, SPRG2 is used as a
scratch register just like SPRG0 and SPRG1. So Declare it as such and fix
the comment which is not valid anymore since that commit.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>