[MTD] mtdoops: allow MTD selection by name

MTD's have both an index number and a name.  Formerly,
the MTD selected for mtdoops was done only by index
number.  With this patch, a name can be used instead.

For example, the kernel command line:

	console=ttyMTD5

selects MTD 5 for mtdoops.  But now this is also possible:

	console=ttyMTD,log

which selects the MTD named "log" for mtdoops.

This has the advantage that partitions can be added or
removed that would affect the MTD index number but not the
name, without having to then change the kernel command line.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Acked-by: Richard Purdie <rpurdie@linux.intel.com>
Acked-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>