add XPA feature

Main changes:
- new field PageGrain.ELPA
- CP0 registers: EntryLo0, EntryLo1, LLAddr and TagLo are extended to 64-bits on MIPS32 core
- new MIPS32r5 MTHC0 and MFHC0 instructions for accessing the upper 32-bits of the above CP0 registers
- EntryLo0/1.PFN and EntryLo0/1.PFNX are concatenated to form the full page frame number