[POWERPC] Support console=comX option for Xen console.

[POWERPC] Support console=comX option for Xen console.
Let Xen/PPC support the console=comX option that Xen/x86 uses, so one
need simply supply CMDLINE="console=com2" on the make invocation to get
proper use of the physical serial port on a JS21.

We do this by pruning all serial devices from the device tree, and then
initializing both com1 and com2 (like arch/x86/setup.c does).

Note that `setenv direct-serial? true; nvupdate' should in theory help
with this, but it does not work at present, and will only be supported
in some future version of SLOF.  Regardless, it is useful to support the
console=comX option, since there will always be environments where it is
difficult to get access to the firmware console.

Tested on JS20 and JS21 blades.

Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>