powerpc/cpm: Clear muram before it is in use.

We need to ensure that MURAM is in a known and cleared out state before
using it as the bootloader could have utilized it from its own purposes
and left it in an unknown state.

If we don't clear it out we've seen issues with UCC ethernet:
* Multi ethernet interfaces can't work simultanously.
* Multi up/down Ethernet interfaces will halt these ports.
* UCC1 RGMII can't work when kernel boots from some hosts.

Signed-off-by: Kai.Jiang <Kai.Jiang@freescale.com>
Signed-off-by: Hongjun Chen <Hong-jun.Chen@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>