regtools: add register access to soc desc

Registers (and variants) can now specify the type of access supported:
- unspecified: for variant means same as register, for register defaults R/W
- read/write
- read only
- write only
Backward compatibility is preserved by setting access to unspecified by default.

Change-Id: I3e84ae18f962a45db62f996a542d08405d05b895

utils/regtools/desc/regs-vsoc2000.xml

@@ -390,6 +390,12 @@
             </instance>
             <register>
                 <width>8</width>
+                <access>read-only</access>
+                <variant>
+                    <type>debug</type>
+                    <offset>4</offset>
+                    <access>write-only</access>
+                </variant>
             </register>
         </node>
     </node>