board: kontron: add sl28 support

Add basic support for the Kontron SMARC-sAL28 board. This includes just
the bare minimum to be able to bring up the board and boot linux.

For now, the Single and Dual PHY variant is supported. Other variants
will fall back to the basic variant.

In particular, there is no watchdog support for now. This means that you
have to disable the default watchdog, otherwise you'll end up in the
recovery bootloader. See the board README for details.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>