fdt: Add a function to get the alias sequence of a node

Aliases are used to provide U-Boot's numbering of devices, such as:

aliases {
	spi0 = "/spi@12330000";
}

spi@12330000 {
	...
}

This tells us that the SPI controller at 12330000 is considered to be the
first SPI controller (SPI 0). So we have a numbering for the SPI node.

Add a function that returns the numbering for a node assume that it exists
in the list of aliases.

Signed-off-by: Simon Glass <sjg@chromium.org>