ARM: dts: bcm283x: Add cells encoding format to firmware bus

With the introduction of 55c7c062 ("ARM: dts: bcm283x: Fix vc4's
firmware bus DMA limitations") the firmware bus has to comply with
/soc's DMA limitations. Ultimately linking both buses to a same
dma-ranges property. The patch (and author) missed the fact that a bus'
#address-cells and #size-cells properties are not inherited, but set to
a fixed value which, in this case, doesn't match /soc's. This, although
not breaking Linux's DMA mapping functionality, generates ugly dtc
warnings.

Fix the issue by adding the correct address and size cells properties
under the firmware bus.

Fixes: 55c7c062 ("ARM: dts: bcm283x: Fix vc4's firmware bus DMA limitations")
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200326134413.12298-1-nsaenzjulienne@suse.de