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>
diff --git a/configs/mx6sabreauto_defconfig b/configs/mx6sabreauto_defconfig
index 418a836..215700b 100644
--- a/configs/mx6sabreauto_defconfig
+++ b/configs/mx6sabreauto_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
+CONFIG_NXP_BOARD_REVISION=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y