arm: imx: mx28: Move MX28 selection to Kconfig

The motivation for moving MX28 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/ts4600_defconfig b/configs/ts4600_defconfig
index aab0737..bc7a576 100644
--- a/configs/ts4600_defconfig
+++ b/configs/ts4600_defconfig
@@ -1,7 +1,8 @@
 CONFIG_ARM=y
-CONFIG_TARGET_TS4600=y
+CONFIG_ARCH_MX28=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_TARGET_TS4600=y
 CONFIG_FIT=y
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 # CONFIG_DISPLAY_CPUINFO is not set