ARM: imx: ddr: Add deskew register programming

Fill is code for programming the DDR_PHY_CMD_DESKEW_CONx registers,
which are optional, but can be used to fill in the byte lane delays.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>