[MTD] [MAPS] don't force uclinux mtd map to be root dev

The cheesy uclinux mtd maps can be used for more than just the root device, so
I think we should drop the forcing.

Also, I feel like this is a policy decision that shouldnt be in the kernel in
the first place.  People who have been lazy and boot with uclinux mtd maps and
dont put root= into their commandline can simply add the appropriate root=
line either into their bootloader or into the compiled in bootargs.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>