[MTD] [MAPS] Document MTD_PHYSMAP module name in kconfig

Help out users by telling them the module name in the Kconfig help when
using the MTD_PHYSMAP option.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>