ARM: uniphier: add dump command for DDR Multi PHY registers

The ProXstream2/PH1-LD6b is integrated with a new IP for DDR PHY
which is not register-compatible with the former SoCs.
Add a new command to support the register dump of this IP.

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