board: rockchip: Add minimal generic RK3399 board

Add a minimal generic RK3399 board that only have eMMC, SDMMC, SPI flash
and USB OTG enabled. This defconfig can be used to boot from eMMC,
SD-card or SPI flash on most RK3399 boards that follow reference board
design.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Christopher Obbard <christopher.obbard@linaro.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index ce53cd5..b06f87b 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -84,6 +84,7 @@
      - FriendlyElec NanoPi M4 (nanopi-m4-rk3399)
      - FriendlyElec NanoPi M4B (nanopi-m4b-rk3399)
      - FriendlyARM NanoPi NEO4 (nanopi-neo4-rk3399)
+     - Generic RK3399 (generic-rk3399)
      - Google Bob (chromebook_bob)
      - Google Kevin (chromebook_kevin)
      - Khadas Edge (khadas-edge-rk3399)