mtd: cleanup Kconfig dependencies

Remove the MTD!=n dependency since that is handled by drivers/mtd/Kconfig.
Simplify the dependency checks for mtd/chips by using if/endif blocks. Remove
all default n since that is the Kconfig default.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>