[POWERPC] iSeries: Make HVC_ISERIES the default

This makes the new iSeries virtual console drivers (nvc_iseries) the
default and prevents viocons being built unless explicitly selected.
Also it makes no sense to have the console as a module.

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