uboot-imx

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>