[MTD] physmap: make physmap compat explicit

The current method for always showing physmap compat address, size, and
width is a bit confusing.  If length is set to 0, then the address and
width are still shown but silently unused.  The physmap code itself
already has logic which sets compat based on length, so just pull that out
and into the Kconfig to make everything clear.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Bryan Wu <Bryan.Wu@analog.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>