ARM: uniphier: drop code for old DT binding

Commit 307d40c5

 ("ARM: uniphier: rework SMP code to support new
System Bus binding") added a new DT binding for SMP code, but still
kept old code for the backward compatibility.

Linux 4.6 was out with both bindings supported, so it should not
hurt to drop the old code now.  Moreover, the mainline code are
currently not used for any of our products, so this change has
no impact on our customers in any way.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>