ARM: omap/fb: move omapfb_reserve_sram to .init.text

This function is only called by omap_detect_sram which lives in .init.text,
too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>