mips: Move MIPS boards into their own files (#14472)

Since the I2000 (Rx2030) and R3030 (Rx3230) boards are so different,
they really belong in their own files. I've kept the original mips.cpp
file for common functions.

There doesn't seem to be enough commonality at this time to make a
common base class worthwhile, perhaps as more boards are implemented
some will be found.

Author: eschaton

src/mame/mame.lst

@@ -28933,9 +28933,11 @@ milton
 raaspec
 vectrex
 
-@source:mips/mips.cpp
+@source:mips/mips_i2000.cpp
 rc2030
 rs2030
+
+@source:mips/mips_r3030.cpp
 rc3230
 rs3230