drivers: ddr: fsl: fix unused-const-variable warnings

Depending on DDR configuration, gcc-6.x will show up unused-const-
variable messages. Use __maybe_unused specifier for all dynamic_odt
variable definitions to remove these warnings.

Memory footprint will not increase as gcc will optimize out unused
constants.

Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>
Signed-off-by: York Sun <york.sun@nxp.com>