dt-bindings: mfd: syscon: Add mt7981-topmisc

dt-bindings: mfd: syscon: Add mt7981-topmisc

This hardware block amongst other things includes a multiplexer for a
high-speed Combo-Phy. This binding allows exposing the multiplexer

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20251115-openwrt-one-network-v4-1-48cbda2969ac@collabora.com
Signed-off-by: Lee Jones <lee@kernel.org>