board: rockchip: Add a common ROCK Pi 4 target

Move ROCK Pi 4 specific board code from the shared evb_rk3399 target
into its own board target and update related defconfigs to use the new
TARGET_ROCKPI4_RK3399 option.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Christopher Obbard <chris.obbard@collabora.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
diff --git a/board/radxa/rockpi4-rk3399/Kconfig b/board/radxa/rockpi4-rk3399/Kconfig
new file mode 100644
index 0000000..d826635
--- /dev/null
+++ b/board/radxa/rockpi4-rk3399/Kconfig
@@ -0,0 +1,15 @@
+if TARGET_ROCKPI4_RK3399
+
+config SYS_BOARD
+	default "rockpi4-rk3399"
+
+config SYS_VENDOR
+	default "radxa"
+
+config SYS_CONFIG_NAME
+	default "rockpi4-rk3399"
+
+config BOARD_SPECIFIC_OPTIONS # dummy
+	def_bool y
+
+endif