Set GDMA1 Frames Destination Port to Port 0 (CPU)

Some U-Boot versions do not initialize MT7620's Frame Engine.
Then it is not possible to receive packets from the network.
Setting GDMA1 Frames Destination Port to Port 0 (CPU) in GDM Forwarding
Configuration register solves this issue.

Submitted by: Hiroki Mori (yamori813@yahoo.co.jp)
Reviewed by: adrian mizhka (previous version)
Differential Revision: https://reviews.freebsd.org/D9301