ARM: uniphier: refactor MEMCONF init code

Currently, memconf-sld3.c and memconf-pxs2.c duplicate the code.

There are 3 patterns in terms of MEMCONF init:
  - DRAM 2 channels: LD4, sLD8, Pro4, Pro5, LD11
  - DRAM 3 channels: sLD3
  - DRAM 3 channels (Ch2 is disable by MEMCONF[21]): Pxs2, LD20

All of them can be moved into a single file by a little more
refactoring.

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