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>

arch/xtensa/Kconfig

@@ -537,7 +537,7 @@ config MEMMAP_CACHEATTR
 	    2: cache bypass,
 	    4: WB cached,
 	    f: illegal.
-	  For ful MMU:
+	  For full MMU:
 	    bit 0: executable,
 	    bit 1: writable,
 	    bits 2..3: