ARM: uniphier: add DRAM init code for ProXstream2/PH1-LD6b

As mentioned in the log of commit 019df879 (ARM: uniphier: add
ProXstream2 and PH1-LD6b support), the DRAM init code was missing
for a long time.  Finally, here it is.  SPL works now.

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