octeontx: do not require cavium BDK node to be present

The cavium,bdk node is a non-standard dt node used by the BDK and
therefore it is removed from the dt before booting Linux. Do not
require this node to exist as it won't for standard dt's.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Stefan Roese <sr@denx.de>