[ARM] MX35: Add register definitions for the i.MX35

This patch moves the stuff common to i.MX31 and i.MX35 to mx3x.h and the
specifics to mx31.h/mx35.h. We can build a kernel which runs on i.MX31 and
i.MX35, so always include mx31.h and mx35.h

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>