ARM: dts: microchip: Drop usb_a9g20-dab-mmx.dtsi

This .dtsi file is not included anywhere in the tree and can't be
tested.

Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20260122202345.3387936-2-robh@kernel.org
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>