scripts: gen_dts_cmake: a small optimization

Using edt.chosen_nodes looks like a simple attribute lookup, but it's
actually calling a property that will create a new list of chosen
nodes every time. Apply a small optimization by only creating the list
once.

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

scripts/dts/gen_dts_cmake.py

diff --git a/scripts/dts/gen_dts_cmake.py b/scripts/dts/gen_dts_cmake.py
index 7d6d1dc..d5434a3 100755
--- a/scripts/dts/gen_dts_cmake.py
+++ b/scripts/dts/gen_dts_cmake.py
@@ -83,8 +83,9 @@
     # macros.bnf for C macros.
 
     cmake_props = []
-    for node in edt.chosen_nodes:
-        path = edt.chosen_nodes[node].path
+    chosen_nodes = edt.chosen_nodes
+    for node in chosen_nodes:
+        path = chosen_nodes[node].path
         cmake_props.append(f'"DT_CHOSEN|{node}" "{path}"')
 
     for node in edt.nodes: