ARM: imx: move cpu definitions into a header

The soc device register code will be moved to drivers/soc/imx/,
the code needs the cpu type definitions. So let's move the cpu
type definitions to a header.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>