ARM: uniphier: set DRAM_SPARSE flag for LD21 boards

Commit 04cd4e72 ("ARM: uniphier: remove DRAM base address from
board parameters") accidentally unset the DRAM_SPARSE flag, and
changed the physical map of the DRAM channels.  Revive the original
behavior.

Fixes: 04cd4e72 ("ARM: uniphier: remove DRAM base address from board parameters")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Shunji Sato <sato.shunji@socionext.com>