board: rockchip: Add minimal generic RK3328 board

Add a minimal generic RK3328 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 RK3328 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 8d7c259..ce53cd5 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -66,6 +66,7 @@
      - FriendlyElec NanoPi R2C Plus (nanopi-r2c-plus-rk3328)
      - FriendlyElec NanoPi R2S (nanopi-r2s-rk3328)
      - FriendlyElec NanoPi R2S Plus (nanopi-r2s-plus-rk3328)
+     - Generic RK3328 (generic-rk3328)
      - Pine64 Rock64 (rock64-rk3328)
      - Radxa ROCK Pi E (rock-pi-e-rk3328)
      - Xunlong Orange Pi R1 Plus (orangepi-r1-plus-rk3328)