meta-openpower: express layer dependencies

meta-openpower has hard dependencies on oe-core, meta-phosphor and
meta-aspeed.  Express those in layer.conf so users can get a helpful
error message if they are missing a required layer from their layer
configuration.  Generally users will look here for this information here
as well, so it helps to have it here.

Change-Id: Icc0e8fb134b281de3d8668b821e6019fcdbfaa84
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/layer.conf b/conf/layer.conf
index 7b3028f..0b3f1a8 100644
--- a/conf/layer.conf
+++ b/conf/layer.conf
@@ -8,4 +8,5 @@
 BBFILE_COLLECTIONS += "openpower-layer"
 BBFILE_PATTERN_openpower-layer := "^${LAYERDIR}/"
 LAYERVERSION_openpower-layer = "1"
+LAYERDEPENDS_openpower-layer = "core phosphor-layer aspeed-layer"
 LAYERSERIES_COMPAT_openpower-layer = "warrior zeus"