mx31pdk: Convert CONFIG_MX31 flag to use Kconfig.

Move CONFIG_MX31 from mx31pdk.h to mx31pdk_defconfig and introduce
necessary Kconfig changes as well.

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index 0f78a63..037e250 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -1,9 +1,10 @@
 CONFIG_ARM=y
 # CONFIG_SPL_USE_ARCH_MEMCPY is not set
 # CONFIG_SPL_USE_ARCH_MEMSET is not set
-CONFIG_TARGET_MX31PDK=y
+CONFIG_ARCH_MX31=y
 CONFIG_SYS_TEXT_BASE=0x87e00000
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_TARGET_MX31PDK=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_SPL=y