ia64: move the ROOT_DEV setup to common code

I'm not sure how useful a platform default ROOT_DEV is these days,
but it pretty sure isn't machvec dependent.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20190813072514.23299-25-hch@lst.de


Signed-off-by: Tony Luck <tony.luck@intel.com>