ARM: uniphier: rework struct uniphier_board_data

ARM: uniphier: rework struct uniphier_board_data

This commit reworks "struct uniphier_board_data" with an array of
DRAM channel data in it.  It will allow further cleanups by means of
"for" statements that iterate over the DDR channels.

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