mx25: Move MX25 selection to Kconfig

mx25: Move MX25 selection to Kconfig

The motivation for moving MX25 selection to Kconfig is to be
able to better handle MX25 specific errata, so that an errata option
can be selected at SoC level instead of board level.

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

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Sebastien Bourdelin <sebastien.bourdelin@savoirfairelinux.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>