irq: Tidy up of-platdata irq support

This function is available but not exported. More generally it does not
really work as intended.

Reimplement it and add a sandbox test too.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/dtb_platdata.py b/tools/dtoc/dtb_platdata.py
index 869c92b..3bb5c68 100644
--- a/tools/dtoc/dtb_platdata.py
+++ b/tools/dtoc/dtb_platdata.py
@@ -62,6 +62,7 @@
 # a phandle property.
 PHANDLE_PROPS = {
     'clocks': '#clock-cells',
+    'interrupts-extended': '#interrupt-cells',
     'gpios': '#gpio-cells',
     'sandbox,emul': '#emul-cells',
     }