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>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index d9a2f5f..c4c4344 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -38,9 +38,10 @@
 	tegra210-p2371-0000.dtb \
 	tegra210-p2571.dtb
 dtb-$(CONFIG_ARCH_UNIPHIER) += \
-	uniphier-ph1-sld3-ref.dtb \
-	uniphier-ph1-pro4-ref.dtb \
 	uniphier-ph1-ld4-ref.dtb \
+	uniphier-ph1-ld6b-ref.dtb \
+	uniphier-ph1-pro4-ref.dtb \
+	uniphier-ph1-sld3-ref.dtb \
 	uniphier-ph1-sld8-ref.dtb
 dtb-$(CONFIG_ARCH_ZYNQ) += zynq-zc702.dtb \
 	zynq-zc706.dtb \