omap-common: SPL: Add CONFIG_SPL_DISPLAY_PRINT / spl_display_print()

Only omap4/5 currently have a meaningful set of display text and overo
had been adding a function to display nothing.  Change how this works to
be opt-in and only turned on for omap4/5 now.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/README b/README
index 5793b0a..309d88d 100644
--- a/README
+++ b/README
@@ -2597,6 +2597,10 @@
 		CONFIG_SYS_SPL_MALLOC_SIZE
 		The size of the malloc pool used in SPL.
 
+		CONFIG_SPL_DISPLAY_PRINT
+		For ARM, enable an optional function to print more information
+		about the running system.
+
 		CONFIG_SPL_LIBCOMMON_SUPPORT
 		Support for common/libcommon.o in SPL binary