MIPS: ralink: dts: mt7628a.dtsi: Add WMAC DT node

This patch adds the WMAC controller description to the MT7628A dtsi file.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@mips.com>
Cc: John Crispin <john@phrozen.org>
Cc: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org