serial: uniphier: drop platform data support

This driver is enabled only for UniPhier SoCs and ARCH_UNIPHIER now
selects OF_CONTROL and SPL_OF_CONTROL.

This driver no longer needs to support platform data configuration.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>