arm: mx6: Make all i.MX6 SoCs user-selectable

arm: mx6: Make all i.MX6 SoCs user-selectable

We have a number of platforms that are a combination of a carrier board
and System-on-Module (SoM) that in turn allows for the board to have
different SoCs on it.  In some cases, this is handled via board-specific
Kconfig options.  In other cases we make use of
CONFIG_SYS_EXTRA_OPTIONS.  This latter case however can lead to invalid
configurations as we will not in turn get options that in Kconfig are
selected by or depend on that setting.

To resolve this, make the SoC option a choice in Kconfig and make boards
depend on what they can support.  This change opens us up for further
clean-ups in the cases where a single CONFIG_TARGET_xxx can support
different SoCs and today they do not, or do not cleanly do so.

Reported-by: Matt Porter <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Fabio Estevam <[email protected]>
Cc: "NXP i.MX U-Boot Team" <[email protected]>
Cc: Soeren Moch <[email protected]>
Cc: Markus Niebel <[email protected]>
Cc: Igor Opaniuk <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Hannes Schmelzer <[email protected]>
Cc: Otavio Salvador <[email protected]>
Cc: Nikita Kiryanov <[email protected]>
Cc: Andreas Geisreiter <[email protected]>
Cc: Ludwig Zenz <[email protected]>
Cc: Lukasz Majewski <[email protected]>
Cc: Akshay Bhat <[email protected]>
Cc: Ken Lin <[email protected]>
Cc: Ian Ray <[email protected]>
Cc: Tim Harvey <[email protected]>
Cc: Jagan Teki <[email protected]>
Cc: Raffaele RECALCATI <[email protected]>
Cc: Simone CIANNI <[email protected]>
Cc: Adam Ford <[email protected]>
Cc: Marcin Niestroj <[email protected]>
Cc: "Eric Bénard" <[email protected]>
Cc: Baruch Siach <[email protected]>
Cc: Jason Liu <[email protected]>
Cc: Ye Li <[email protected]>
Cc: Eric Nelson <[email protected]>
Cc: Troy Kisky <[email protected]>
Cc: Peng Fan <[email protected]>
Cc: Parthiban Nallathambi <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: "Sébastien Szymanski" <[email protected]>
Cc: Christian Gmeiner <[email protected]>
Cc: Niel Fourie <[email protected]>
Cc: Martyn Welch <[email protected]>
Cc: Richard Hu <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Boris Brezillon <[email protected]>
Cc: Arkadiusz Karas <[email protected]>
Cc: Breno Lima <[email protected]>
Cc: Francesco Montefoschi <[email protected]>
Cc: Silvio Fricke <[email protected]>
Tested-by: Matt Porter <[email protected]> [colibri_imx6]
Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Marcin Niestroj <[email protected]>