doc: sifive: fu540: Add description for OpenSBI generic platform

OpenSBI generic platform support provides platform specific
functionality based on the FDT passed by previous booting stage.

Depends on OpenSBI commit:
platform: Add generic FDT based platform support
(sha1: f1aa9e54e00006ae70aeac638d5b75093520f65d)

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/doc/board/sifive/fu540.rst b/doc/board/sifive/fu540.rst
index 15c2eaa..f7c2c9f 100644
--- a/doc/board/sifive/fu540.rst
+++ b/doc/board/sifive/fu540.rst
@@ -20,17 +20,6 @@
 4. SiFive SPI Driver.
 5. MMC SPI Driver for MMC/SD support.
 
-TODO:
-
-1. U-Boot expects the serial console device entry to be present under /chosen
-   DT node. Without a serial console U-Boot will panic. Example:
-
-.. code-block:: none
-
-   chosen {
-        stdout-path = "/soc/serial@10010000:115200";
-   };
-
 Booting from MMC using FSBL
 ---------------------------
 
@@ -60,7 +49,7 @@
 
 .. code-block:: none
 
-make PLATFORM=sifive/fu540 FW_PAYLOAD_PATH=<path to u-boot-dtb.bin>
+	make PLATFORM=generic FW_PAYLOAD_PATH=<path to u-boot-dtb.bin>
 
 More detailed description of steps required to build FW_PAYLOAD firmware
 is beyond the scope of this document. Please refer OpenSBI documenation.