ARM: UniPhier: do not compile unnecessary objects

It is true that unused functions are removed from the ELF image
by the compiler's garbage collection but relying on it too much
does not look nice.
Currently, the build is taking more than it should.

Refactor the makefiles to compile only files that are really needed.
CONFIG_SOC_INIT and CONFIG_DRAM_INIT are no longer needed by the
optimization.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>