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>