arm/imx2x: split i.MX21/i.MX27 register definitions

* define new CONFIG_ARCH_MX21 (this one is currently mutually exclusive to
  CONFIG_ARCH_MX27, but this might change)
* splits one header file. Memory definitions, interrupt sources,
  DMA channels are split into common part, i.MX27 specific and i.MX21
  specific.
* guard access to UART5/UART6, which don't exist on i.MX21

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>