ARM: uniphier: use gd->bd->bi_dram for memory reserve on LD20 SoC

ARM: uniphier: use gd->bd->bi_dram for memory reserve on LD20 SoC

For LD20 SoC, the last 64 byte of each DRAM bank is used for the
dynamic training of DRAM PHY.  The regions must be reserved in DT to
prevent the kernel from using them.  Now gd->bd->bi_dram reflects
the actual memory banks.  Just use it instead of getting access to
the board parameters.

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