doc: Clean up SYS_MALLOC_SIMPLE

Move the useful help to Kconfig.

Drop mention of CONFIG_SYS_MALLOC_SIMPLE since it doesn't exist.

Correct a 'CONFIGSYS_MALLOC_F_LEN' typo

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/cmd/bdinfo.rst b/doc/usage/cmd/bdinfo.rst
index b287d0f..5261085 100644
--- a/doc/usage/cmd/bdinfo.rst
+++ b/doc/usage/cmd/bdinfo.rst
@@ -106,7 +106,7 @@
 
 Early malloc usage
     amount of memory used in the early malloc memory and its maximum size
-    as defined by CONFIGSYS_MALLOC_F_LEN
+    as defined by CONFIG_SYS_MALLOC_F_LEN
 
 Configuration
 -------------