imxfb: add support for i.MX25

The commit also introduces the HAVE_FB_IMX config that is to be selected from
the MX25 platform config. Once this commit gets merged, the two other i.MX
archs, ARCH_MX1 and ARCH_MX2, should follow this one.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>