mtd: update help of CONFIG_MTD_OF_PARTS

The help text of CONFIG_MTD_OF_PARTS refers to additional documentation
in booting-without-of.txt but this documentation was moved to another
file in commit efcc2da3 (Stefan Roese:
Factor MTD physmap bindings out of booting-without-of). This updates the
help text to point to the right place.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
[Brian: fixed doc reference]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>