mx6: add structs for mmdc and ddr iomux registers

Add memory-mapped structures for MMDC iomux and configuration. Note that whi
the MMDC configuration registers are common between the IMX6DQ
(IMX6DUAL/IMX6QUAD) and IMX6SDL (IMX6SOLO/IMX6DUALLITE) types the iomux
registers differ. This requires two sets of structures.

Add structures to describe DDR3 device information, system information
(memory layout, etc), and MMDC calibration registers that can be used to
configure the MMDC dynamically.

We define these structures for SPL builds instead of including mx6q-ddr.h an
mx6dl-ddr.h which use the same namespace and are only useful for imximage cf
files.

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>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>