imx: sys_proto: Add a prototype for board_mmc_get_env_dev()

When compiling with W=1 the following warning is observed:

board/freescale/mx6sabresd/mx6sabresd.c:266:5: warning:
no previous prototype for ‘board_mmc_get_env_dev’
[-Wmissing-prototypes] int board_mmc_get_env_dev(int devno)

Remove this warning by adding the function prototype into sys_proto.h file.

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>