ARM: uniphier: introduce flags to adjust DRAM timing for LD20/LD21

ARM: uniphier: introduce flags to adjust DRAM timing for LD20/LD21

Unfortunately, this SoC needs per-board adjustment between clock
and address/command lines.  This flag will be passed to the DRAM
init function and used for compensating the difference of DRAM
timing parameters.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>