[POWERPC] iseries: A new iSeries console

This driver uses the hvc_console.c infrastructure that is used by the
pSeries virtual and RTAS consoles.  This will allow us to make viocons.c
obsolete and is another step along the way to a combined kernel (as
viocons could not coexist with CONFIG_VT).

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>