fdt: Add new fdt_set_node_status & fdt_set_status_by_alias helpers

Add common function fdt_set_node_status() to assist in various locations
that we set a nodes status.  This function utilizes the status values
that are part of the EPAPR spec (on power.org).

fdt_set_status_by_alias() is based on fdt_set_node_status() but uses an
alias string to identify the node to update.

We also add some shortcut functions to help the common cases of setting
"okay" and "disabled":

	fdt_status_okay()
	fdt_status_disabled()
	fdt_status_okay_by_alias()
	fdt_status_disabled_by_alias()

Finally, we fixup the corenet_ds ethernet code which previously had
a function by the same name that can be replaced with the new helpers.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>