rockchip: Add documentation for VBE

Now that VBE is running at a basic level on rk3399, add mention of it in
the documentation.

doc/board/rockchip/rockchip.rst

@@ -519,6 +519,19 @@ config-flash.ini:
         [OUTPUT]
         PATH=RK30xxLoader_uboot.bin
 
+Verified Boot for Embedded (VBE)
+--------------------------------
+
+VBE is supported on rk3399 at present, with SDRAM being set up in SPL. The full
+A/B/recovery flow is supported in U-Boot.
+
+To build this, use the rk3399-generic board, setting CONFIG_OF_LIST to the list
+of boards you want to support. All boards must use the same SDRAM type.
+
+VBE uses internal SRAM (IRAM) for the TPL, VPL and early SPL phases. The stack
+stop is near the top of this (0xff8eff00) with the 256-byte bloblist immediately
+following.
+
 TODO
 ----