mx6: add mmdc configuration for MX6Q/MX6DL

mx6: add mmdc configuration for MX6Q/MX6DL

- add function for configuring iomux based on board-specific regs
- add function for configuring mmdc based on board-specific and
  chip-specific data

Cc: Stefan Roese <sr@denx.de>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Andy Ng <andreas2025@gmail.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>
Cc: Tapani Utriainen <tapani@technexion.com>
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>