doc, spl, am335x: update am335x-network

on the shc board we see when booting in net boot mode,
that the ROM bootloader sends "AM335x ROM" as
vendor-class-identifier. U-Boots doc says "DM814x ROM".

So, add the info to the doc, that there is also
"AM335x ROM" possible.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>

doc/SPL/README.am335x-network

@@ -66,6 +66,10 @@ subnet 192.168.8.0 netmask 255.255.255.0 {
   }
 }
 
+  May the ROM bootloader sends another "vendor-class-identifier"
+  on the shc board with an AM335X it is:
+  "AM335x ROM"
+
   2. Setup TFTP server.
      Install TFTP server and put image files to it's root directory
 (likely /tftpboot or /var/lib/tftpboot or /srv/tftp). You will need