Simplify the *CacheOp enums.

This adds a CacheOpEncoder class to encode InstructionCacheOp and DataCacheOp
values, in the same way as SystemRegister.

I've also replaced the hexadecimal field values with decimals, since this is
easier to read and also matches the table in the Arm ARM.

Change-Id: I05bb334ebce77f2b3847fb70afdfc30aaf798e30