Select default U-Boot and SPL text base for the i.MX6 SoC. The U-Boot
and SPL text base is picked as the one used by various i.MX6 boards.
Update all the boards.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>