fixing typo error in README file. CPU15 -> CP15

It looks typo error.
Not CPU15, CP15(CoProcessor15)

Signed-off-by: yeongjun Kim <iam.yeongjunkim@gmail.com>
diff --git a/README b/README
index 9ecac55..ecbd07f 100644
--- a/README
+++ b/README
@@ -4811,7 +4811,7 @@
 
 - CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 		[ARM926EJ-S only] This allows just the call to lowlevel_init()
-		to be skipped. The normal CPU15 init (such as enabling the
+		to be skipped. The normal CP15 init (such as enabling the
 		instruction cache) is still performed.
 
 - CONFIG_SPL_BUILD