ARM: dts: turris-omnia: fix hardware buffer management

Hardware buffer management has never worked on the Turris Omnia, as the
required MBus window hadn't been reserved. Fix thusly.

Fixes: 018b88ee ("ARM: dts: turris-omnia: enable HW buffer management")

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
Reviewed-by: Marek Behún <kabel@kernel.org>
Tested-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>