mx6memcal - a tool for calibrating DDR on i.MX6 boards.

The mx6memcal board isn't a real board, but a tool for use in bring-up of
new i.MX6 board designs.

It provides a similar function to the tool from NXP([1]) with a number
of advantages:

1. It's open-source, so it's easier to change if needed.
   Typical reasons for needing to change include the use of alternate
   UARTs and PMIC initialization.
2. It produces an image that's directly loadable with imx_usb [2] or
   SB_LOADER.exe [3].
   The NXP tool requires either a cumbersome JTAG connection that
   makes running the DDR very slow or a working U-Boot image that
   suffers from a chicken-and-egg problem (i.e. where do you get the
   DDR parameters for U-Boot?).
3. It doesn't prompt for parameters, so it's much faster to gather
   data from multiple boards.
4. Parameters to the calibration process can be chosen through
   'make menuconfig'.

When booted, the mx6memcal board will run the DDR calibration
routines and display the result in a form suitable for cut and
paste into struct mx6_mmdc_calibration. It can also optionally
produce output in a form usable in a DCD-style .cfg file.

Selections in Kconfig allow most system design settings to be chosen:

1. The UART number and pad configuration for the UART. Options
   include support for the most frequent reference designs on
   i.MX6DQ/SDL (SABRE Lite and SABRESD designs).
2. The memory bus width (64 and 32-bit)
3. The number of chip-selects in use
4. The type of DDR (DDR3 or LPDDR2). Note that LPDDR2 support
   is incomplete as of this writing.
5. The type of DDR chips in use. This selection allows re-use of common
   parts and four DDR3 and two LPDDR2 parts are currently defined
6. The On-die termination value for the DRAM lines
7. The DRAM drive strength
8. The RTT_NOM and RTT_WR termination settings
9. RALAT/WALAT latency values

References:
[1] - NXP DDR Stress Test Tool - https://community.nxp.com/docs/DOC-105652
[2] - Boundary Devices imx_usb_loader
      https://github.com/boundarydevices/imx_usb_loader
[3] - Use of SB_Loader.exe
      https://boundarydevices.com/windows-users-and-unbricking
