imx: Add a common way for detecting NXP boards revision

NXP development boards based on i.MX6/i.MX7 contain the board
revision information stored in the fuses.

Introduce a common function that can be shared by different boards and
convert mx6sabreauto to use this new mechanism.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

arch/arm/include/asm/mach-imx/sys_proto.h

@@ -109,6 +109,9 @@ void imx_wdog_disable_powerdown(void);
 
 int board_mmc_get_env_dev(int devno);
 
+int nxp_board_rev(void);
+char nxp_board_rev_string(void);
+
 /*
  * Initializes on-chip ethernet controllers.
  * to override, implement board_eth_init()