OMAP3 SPL: Provide weak omap_rev_string

We add an weak version of omap_rev_string in omap-common/spl.c
and while at it drop the omap3 version.  Move the prototype over
to <asm/omap_common.h> with the other SPL functions.

Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>

arch/arm/include/asm/omap_common.h

@@ -77,6 +77,7 @@ u32 omap_boot_mode(void);
 
 /* SPL common function s*/
 void spl_parse_image_header(const struct image_header *header);
+void omap_rev_string(char *omap_rev_string);
 
 /* NAND SPL functions */
 void spl_nand_load_image(void);