ADS5121 cleanup compile warnings

board/ads5121/iopin.c
    Replace bit fields in struct iopin_t with a single
    field and intialize it via plain old macros.
    This fixes the type pun warnings and makes the code
    more readable.

board/ads5121/ads5121.c
    Add include iopin.h to ads5121.c for the iopin_initialize
    prototype.

    Add an extern void ads5121_diu_init(void)

Signed-off-by: John Rigby <jrigby@freescale.com>