xtensa: fix Kconfig typo

Correct trivial typo (ful -> full).

Fixes: 76743c0e

 ("xtensa: move kernel memory layout to platform options")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: linux-xtensa@linux-xtensa.org
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>