Use the cell-index property as the unit number if available.

Summary:
NXP/Freescale, among others, includes an optional cell-index property
on nodes to denote the SoC block number of the node. This can be useful if, for
example, a node is disabled or nonexistent in the fdt, or the blocks are not
organized in address-sorted order. For instance, on the P1022, DMA2 is located
at CCSR offset 0xC000, while DMA1 is located at 0x21000.

Reviewed By: jmcneill

Differential Revision: https://reviews.freebsd.org/D8054