devicetree: update DT_PROP_LEN documentation about ranges and dma-ranges

Like reg and interrupts, using DT_PROP_LEN doesn't make sense since
the ranges & dma-ranges block lengths and values depends on the DT
node #address-cells, #size-cells and parents #address-cells value.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>