ARM: dts: uniphier: add ProXstream2 and PH1-LD6b SoC/board support

Initial version of DTSI for ProXstream2 and PH1-LD6b and DTS for
PH1-LD6b reference board.

Import from Linux with some adjustments:
  - Use SPDX-License-Identifier
  - Add clock-frequency to serial nodes
  - Drop unusable nodes from -ref.dts

While I am here, sort Makefile entries alphabetically.

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