Power: Fix MaxMiscDestRegs which was set to zero

This patch fixes a failing compilation caused by MaxMiscDestRegs being
zero. According to gcc 4.6, the result is a comparison that is always
false due to limited range of data type.

src/arch/power/registers.hh

@@ -38,7 +38,10 @@ namespace PowerISA {
 
 using PowerISAInst::MaxInstSrcRegs;
 using PowerISAInst::MaxInstDestRegs;
-using PowerISAInst::MaxMiscDestRegs;
+
+// Power writes a misc register outside of the isa parser, so it can't
+// be detected by it. Manually add it here.
+const int MaxMiscDestRegs = PowerISAInst::MaxMiscDestRegs + 1;
 
 typedef uint8_t RegIndex;