arch/powerpc/cpu/mpc85xx/cpu_init.c: Fix GCC 4.6 build warning

Fix:

cpu_init.c: In function 'cpu_init_r':
cpu_init.c:320:7: warning: variable 'l2srbar' set but not used [-Wunused-but-set-variable]

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>