edtlib: move vendor-prefixes.txt parsing in here

Take this helper from the doc tooling and expose it as API.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

doc/_scripts/gen_devicetree_rest.py

@@ -73,20 +73,7 @@ class VndLookup:
         vnd2vendor = {
             None: GENERIC_OR_VENDOR_INDEPENDENT,
         }
-        with open(vendor_prefixes, 'r', encoding='utf-8') as f:
-            for line in f:
-                line = line.strip()
-
-                if not line or line.startswith('#'):
-                    # Comment or empty line.
-                    continue
-
-                # Other lines should be in this form:
-                #
-                # <vnd><TAB><vendor>
-                vnd_vendor = line.split('\t', 1)
-                assert len(vnd_vendor) == 2, line
-                vnd2vendor[vnd_vendor[0]] = vnd_vendor[1]
+        vnd2vendor.update(edtlib.load_vendor_prefixes_txt(vendor_prefixes))
 
         logger.info('found %d vendor prefixes in %s', len(vnd2vendor) - 1,
                     vendor_prefixes)

scripts/dts/python-devicetree/src/devicetree/edtlib.py

@@ -1976,6 +1976,31 @@ class PropertySpec:
 class EDTError(Exception):
     "Exception raised for devicetree- and binding-related errors"
 
+#
+# Public global functions
+#
+
+
+def load_vendor_prefixes_txt(vendor_prefixes):
+    """Load a vendor-prefixes.txt file and return a dict
+    representation mapping a vendor prefix to the vendor name.
+    """
+    vnd2vendor = {}
+    with open(vendor_prefixes, 'r', encoding='utf-8') as f:
+        for line in f:
+            line = line.strip()
+
+            if not line or line.startswith('#'):
+                # Comment or empty line.
+                continue
+
+            # Other lines should be in this form:
+            #
+            # <vnd><TAB><vendor>
+            vnd_vendor = line.split('\t', 1)
+            assert len(vnd_vendor) == 2, line
+            vnd2vendor[vnd_vendor[0]] = vnd_vendor[1]
+    return vnd2vendor
 
 #
 # Private global functions