imx: Move some header files from arch-mxs to imx-common

The following headers are moved to a i.MX common location:

- regs-common.h
- regs-apbh.h
- regs-bch.h
- regs-gpmi.h
- dma.h

This way this header can be re-used also by other i.MX platforms.
For example the i.MX6 which will need it for the upcoming NAND
support.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>