i.MX Framebuffer: Use readl/writel instead of direct pointer deref

This patch prepares the current i.MX1 framebuffer driver for usage in the
whole i.MX family. It switches to readl/writel for register accesses.
Also it moves the register definitions to the driver where they belong.

Acked-by: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Juergen Beisert <j.beisert@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>