arch: arm: dts: kirkwood-rd88f6281: Remove disabled marvell,dsa reference

marvell,dsa properties has been removed from kirkwood-rd88f6281.dtsi
while cleanuping the dsa binding, but the dsa reference in
kirkwood-rd88f6281-z0.dts has been missed causing the following errors:

arch/arm/boot/dts/kirkwood-rd88f6281-z0.dts:21.4-16: Warning (reg_format): /dsa/switch@0:reg: property has invalid length (8 bytes) (#address-cells == 2, #size-cells == 1)
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dts:23.5-15: Warning (reg_format): /dsa/switch@0/port@4:reg: property has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dtb: Warning (pci_device_bus_num): Failed prerequisite 'reg_format'
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dtb: Warning (i2c_bus_reg): Failed prerequisite 'reg_format'
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dtb: Warning (spi_bus_reg): Failed prerequisite 'reg_format'
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dts:20.12-26.5: Warning (avoid_default_addr_size): /dsa/switch@0: Relying on default #address-cells value
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dts:20.12-26.5: Warning (avoid_default_addr_size): /dsa/switch@0: Relying on default #size-cells value
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dts:22.11-25.6: Warning (avoid_default_addr_size): /dsa/switch@0/port@4: Relying on default #address-cells value
arch/arm/boot/dts/kirkwood-rd88f6281-z0.dts:22.11-25.6: Warning (avoid_default_addr_size): /dsa/switch@0/port@4: Relying on default #size-cells value

So remove the dsa reference too in order to fix this issue.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>