Add Zilsd extension YAML

Add support for the Zilsd extension, which introduces load/store pair instructions for RV32.
These instructions allow efficient 64-bit memory operations using register pairs.
Ensure proper alignment rules, trap handling, and atomicity requirements.
Improve memory efficiency and reduce instruction count for doubleword operations.

Author: Shehrozkashif

arch/ext/Zilsd.yaml

@@ -0,0 +1,13 @@
+$schema: "ext_schema.json#"
+kind: extension
+name: Zilsd
+long_name: Load/Store Pair for RV32
+description: |
+  This specification adds load and store instructions using register pairs. It does so by reusing existing instruction encodings which are RV64-only. The specification defines 32-bit encodings.
+  Load and store instructions will use the same definition of even-odd pairs as defined by the Zdinx extension.
+  The extension improves static code density, by replacing two separate load or store instructions with a single one. In addition, it can provide a performance improvement for implementations that can make use of a wider than XLEN memory interface.
+type: unprivileged
+versions:
+  - version: "1.0"
+    state: ratified
+    ratification_date: "2025-02"