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>

README

@@ -1281,24 +1281,6 @@ Configuration Settings:
 - CONFIG_SYS_MALLOC_LEN:
 		Size of DRAM reserved for malloc() use.
 
-- CONFIG_SYS_MALLOC_F_LEN
-		Size of the malloc() pool for use before relocation. If
-		this is defined, then a very simple malloc() implementation
-		will become available before relocation. The address is just
-		below the global data, and the stack is moved down to make
-		space.
-
-		This feature allocates regions with increasing addresses
-		within the region. calloc() is supported, but realloc()
-		is not available. free() is supported but does nothing.
-		The memory will be freed (or in fact just forgotten) when
-		U-Boot relocates itself.
-
-- CONFIG_SYS_MALLOC_SIMPLE
-		Provides a simple and small malloc() and calloc() for those
-		boards which do not use the full malloc in SPL (which is
-		enabled with CONFIG_SPL_SYS_MALLOC).
-
 - CFG_SYS_BOOTMAPSZ:
 		Maximum size of memory mapped by the startup code of
 		the Linux kernel; all data that must be processed by