arm64: dts: mediatek: mt8186: Move wakeup to MTU3 to get working suspend

The current DT has the wakeup-source and mediatek,syscon-wakeup
properties in the XHCI nodes, which configures USB wakeup after powering
down the XHCI hardware block. However, since the XHCI controller is
behind an MTU3 (USB3 DRD controller), the MTU3 only gets powered down
after USB wakeup has been configured, causing the system to detect a
wakeup, and results in broken suspend support as the system resumes
immediately.

Move the wakeup properties to the MTU3 nodes so that USB wakeup is only
enabled after the MTU3 has powered down.

With this change in place, it is possible to suspend and resume, and
also to wakeup through USB, as tested on the Google Steelix (Lenovo 300e
Yoga Chromebook Gen 4).

Fixes: f6c3e61c ("arm64: dts: mediatek: mt8186: Add MTU3 nodes")
Reported-by: Wojciech Macek <wmacek@google.com>
Suggested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>