ARM: uniphier: support ProXstream2, PH1-LD6b boards in single defconfig

These boards are similar enough to be supported in a single defconfig
file.  Distinguish one from another by "DEVICE_TREE" from the command
line.  The how-to-build in doc/README.uniphier should be also updated.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/doc/README.uniphier b/doc/README.uniphier
index 68cc05d..7562e6f 100644
--- a/doc/README.uniphier
+++ b/doc/README.uniphier
@@ -48,14 +48,18 @@
     $ make ph1_pro5_defconfig
     $ make CROSS_COMPILE=arm-linux-gnueabi-
 
-ProXstream2:
-    $ make pxs2_defconfig
-    $ make CROSS_COMPILE=arm-linux-gnueabi-
+ProXstream2 Gentil board:
+    $ make uniphier_pxs2_ld6b_defconfig
+    $ make CROSS_COMPILE=arm-linux-gnueabi- DEVICE_TREE=uniphier-proxstream2-gentil
 
-PH1-LD6b:
-    $ make ph1_ld6b_defconfig
+ProXstream2 Vodka board:
+    $ make uniphier_pxs2_ld6b_defconfig
     $ make CROSS_COMPILE=arm-linux-gnueabi-
 
+PH1-LD6b reference board:
+    $ make uniphier_pxs2_ld6b_defconfig
+    $ make CROSS_COMPILE=arm-linux-gnueabi- DEVICE_TREE=uniphier-ph1-ld6b-ref
+
 You may wish to change the "CROSS_COMPILE=arm-linux-gnueabi-"
 to use your favorite compiler.