serial: uniphier: use register macros instead of structure

After all, I am not a big fan of using a structure to represent the
hardware register map.

You do not need to know the entire register map.

Add only necessary register macros.

Use FIELD_PREP() instead of maintaining a pair of shift and mask.

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