i.MX6 (DQ/DLS): use macros for mux and pad declarations

i.MX6 (DQ/DLS): use macros for mux and pad declarations

This allows the use of either or both declarations from
the files mx6q_pins.h and mx6dl_pins.h.

All board files should include <asm/arch/mx6-pins.h>
with one of the following defined in boards.cfg
    MX6Q   - for boards targeting i.MX6Q or i.MX6D
    MX6DL  - for boards targeting i.MX6DL
    MX6S   - for boards targeting i.MX6S
    MX6QDL - for boards that support any of the above with
             run-time detection

Pad declarations will be MX6_PAD_x for single-variant boards
and MX6Q_PAD_x and MX6DL_PAD_x for boards supporting both
processor classes.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Stefano Babic <sbabic@denx.de>