pseries: Populate "/chosen/linux,stdout-path" in the FDT

pseries: Populate "/chosen/linux,stdout-path" in the FDT

There is a device tree property "/chosen/linux,stdout-path" which indicates
which device should be used as stdout - ie. "the console".

Currently we don't specify anything, which means both firmware and Linux
choose something arbitrarily. Use the routine we added in the last patch
to pick a default vty and specify it as stdout.

Currently SLOF doesn't use the property, but we are hoping to update it
to do so.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
(cherry picked from commit 68f3a94c64bbaaf8c7f2daa70de1b5d87a432f86)