ARM: mx6: ddr: use Kconfig for inclusion of DDR calibration routines

The DDR calibration routines are gated by conditionals for the
i.MX6DQ SOCs, but with the use of the sysinfo parameter, these
are usable on at least i.MX6SDL and i.MX6SL variants with DDR3.

Also, since only the Novena board currently uses the dynamic
DDR calibration routines, these routines waste space on other
boards using SPL.

Add a KConfig entry to allow boards to selectively include the
DDR calibration routines.

Signed-off-by: Eric Nelson <eric@nelint.com>