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 <mporter@konsulko.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: "NXP i.MX U-Boot Team" <uboot-imx@nxp.com>
Cc: Soeren Moch <smoch@web.de>
Cc: Markus Niebel <Markus.Niebel@tq-group.com>
Cc: Igor Opaniuk <igor.opaniuk@toradex.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Andreas Geisreiter <ageisreiter@dh-electronics.de>
Cc: Ludwig Zenz <lzenz@dh-electronics.de>
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Akshay Bhat <akshaybhat@timesys.com>
Cc: Ken Lin <Ken.Lin@advantech.com.tw>
Cc: Ian Ray <ian.ray@ge.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Raffaele RECALCATI <raffaele.recalcati@bticino.it>
Cc: Simone CIANNI <simone.cianni@bticino.it>
Cc: Adam Ford <aford173@gmail.com>
Cc: Marcin Niestroj <m.niestroj@grinn-global.com>
Cc: "Eric Bénard" <eric@eukrea.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Jason Liu <jason.hui.liu@nxp.com>
Cc: Ye Li <ye.li@nxp.com>
Cc: Eric Nelson <eric@nelint.com>
Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Parthiban Nallathambi <parthiban@linumiz.com>
Cc: Marek Vasut <marex@denx.de>
Cc: "Sébastien Szymanski" <sebastien.szymanski@armadeus.com>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Niel Fourie <lusus@denx.de>
Cc: Martyn Welch <martyn.welch@collabora.com>
Cc: Richard Hu <richard.hu@technexion.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Boris Brezillon <bbrezillon@kernel.org>
Cc: Arkadiusz Karas <arkadiusz.karas@somlabs.com>
Cc: Breno Lima <breno.lima@nxp.com>
Cc: Francesco Montefoschi <francesco.montefoschi@udoo.org>
Cc: Silvio Fricke <open-source@softing.de>
Tested-by: Matt Porter <mporter@konsulko.com> [colibri_imx6]
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Marcin Niestroj <m.niestroj@grinn-global.com>