spl: Add spl_early_init()

At present malloc_base/_limit/_ptr are not initialised in spl_init() when
we call spl_init() in board_init_f(). This is due to a recent change aimed
at avoiding overwriting the malloc area set up on some boards by
spl_relocate_stack_gd().

However if CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN is not defined, we now
skip setting up the memory area in spl_init() which is obviously wrong.

To fix this, add a new function spl_early_init() which can be called in
board_init_f().

Fixes: b3d2861e (spl: Remove overwrite of relocated malloc limit)
Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Rewrote spl_{,early_}init() to avoid duplicate code:
Rewrite/expand commit message:
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Eddie Cai <eddie.cai.linux@gmail.com>

include/asm-generic/global_data.h

@@ -127,5 +127,6 @@ typedef struct global_data {
 #define GD_FLG_SKIP_RELOC	0x00800	/* Don't relocate		   */
 #define GD_FLG_RECORD		0x01000	/* Record console		   */
 #define GD_FLG_ENV_DEFAULT	0x02000 /* Default variable flag	   */
+#define GD_FLG_SPL_EARLY_INIT	0x04000 /* Early SPL init is done	   */
 
 #endif /* __ASM_GENERIC_GBL_DATA_H */