ARM: mx6: ddr: Add support for iMX6SX

This patch adds support for iMX6SX MMDC into the DDR calibration
code. The only difference between MX6DQ and MX6SX is that the SX
has 2 SDQS registers, while the DQ has 8.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Eric Nelson <eric@nelint.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Eric Nelson <eric@nelint.com>