powerpc: e6500: Lock/unlock L2 cache instead of L1 as init_ram

MPC85xx has been using locked L1 cache as init_ram. L1 cache is a write
through cache on E6500. L2 cache is enabled to to hold the data. This
patch locks/unlocks L2 cache to ensure no data cast out from L2 cache.

Signed-off-by: York Sun <yorksun@freescale.com>
Reported-by: Jeffery Zhu <Jefferry.Zhu@freescale.com>