Add MIPS64R6 support

#### List of added features: ####
* support for MIPS64R6 and MIPS32R6 instruction set
 - new CPU definitions: mips64r6-generic and mips32r6-generic
* pre-R6 features which became mandatory in R6:
 - KScratch registers
 - Read-Inhibit and Execute-Inhibit page protection bits
 - TLB Invalidate (TLBINV and TLBINVF instructions)
 - BadInstr and BadInstrP registers
* new R6 features:
 - forbidden slot
 - Config5.SBRI
 - reserved fields / registers


#### Limitations: ####
- no support for misaligned memory accesses
- no support for FIR.HAS2008

#### Documentation: ####
Volume I: Introduction to the MIPS64 Architecture v6.01
Volume II: The MIPS64 Instruction Set v6.00
Volume III: The MIPS64 and microMIPS64 Privileged Resource Architecture v6.00
Available at: http://www.imgtec.com/mips/architectures/mips64.asp