arm: imx: mx23: Move MX23 selection to Kconfig

The motivation for moving MX23 selection to Kconfig is to be able
to better handle NAND MXS selection through Kconfig.

This selection method also aligns with the way other i.MX SoCs are
selected in U-Boot.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
diff --git a/configs/mx23_olinuxino_defconfig b/configs/mx23_olinuxino_defconfig
index 628c11d..d70ca52 100644
--- a/configs/mx23_olinuxino_defconfig
+++ b/configs/mx23_olinuxino_defconfig
@@ -1,8 +1,9 @@
 CONFIG_ARM=y
-CONFIG_TARGET_MX23_OLINUXINO=y
+CONFIG_ARCH_MX23=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_TARGET_MX23_OLINUXINO=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y