docs: platform/generic: Add details about stdout-path DT property

The generic platform will try to select serial console based on the
stdout-path DT property in /chosen DT node hence we document this.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

docs/platform/generic.md

@@ -23,6 +23,8 @@ Users of the generic FDT platform will have to ensure that:
    based on their platform requirements
 2. The FDT passed by previous booting stage has DT compatible strings and
    DT properties in sync with the FDT based drivers under lib/utils directory
+3. The FDT must have "stdout-path" DT property in the "/chosen" DT node when
+   a platform has multiple serial ports or consoles
 
 To build the platform-specific library and firmware images, provide the
 *PLATFORM=generic* parameter to the top level `make` command.