Compile some MIPS devices only once

Move CPU specific declarations to a separate file.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/mips_mipssim.c b/hw/mips_mipssim.c
index 98d1a01..a747de5 100644
--- a/hw/mips_mipssim.c
+++ b/hw/mips_mipssim.c
@@ -26,6 +26,7 @@
  */
 #include "hw.h"
 #include "mips.h"
+#include "mips_cpudevs.h"
 #include "pc.h"
 #include "isa.h"
 #include "net.h"