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/arch/arm/cpu/armv7/omap-common/hwinit-common.c b/arch/arm/cpu/armv7/omap-common/hwinit-common.c
index 459ebb5..4d7ef5c 100644
--- a/arch/arm/cpu/armv7/omap-common/hwinit-common.c
+++ b/arch/arm/cpu/armv7/omap-common/hwinit-common.c
@@ -92,6 +92,11 @@
 {
 	boot_params_ptr = (u32 *) &boot_params;
 }
+
+void spl_display_print(void)
+{
+	omap_rev_string();
+}
 #endif
 
 /*