powerpc/b4860: Renamed the L2 caches

To make provision for more than one L2 caches in the system, change the
name from L2 to L2_1; same as in T4 platforms.
* Also remove the L2 entry from common file
  "arch/powerpc/boot/dts/fsl/b4si-post.dtsi"
  Keep them only in separate files for b4860 and b4420.

Signed-off-by: Shaveta Leekha <shaveta@freescale.com>
Signed-off-by: Poonam Aggrwal <poonam.aggrwal@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>