lrdp-v2: Introduce read_s64

Add a function for reading a signed 64 bit value from a ramdump.

Change-Id: Ic7e5a8a7cb9d5fd551ffb2c3add2499a25aff540

linux-ramdump-parser-v2/ramdump.py

@@ -1274,6 +1274,11 @@ class RamDump():
         s = self.read_string(addr_or_name, '<?', virtual, cpu)
         return s[0] if s is not None else None
 
+    def read_s64(self, addr_or_name, virtual=True, cpu=None):
+        """returns a value guaranteed to be 64 bits"""
+        s = self.read_string(addr_or_name, '<q', virtual, cpu)
+        return s[0] if s is not None else None
+
     def read_u64(self, addr_or_name, virtual=True, cpu=None):
         """returns a value guaranteed to be 64 bits"""
         s = self.read_string(addr_or_name, '<Q', virtual, cpu)