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>