MIPS: unaligned: Add DSP lwx & lhx missaligned access support
Add handling of missaligned access for DSP load instructions
lwx & lhx.
Since DSP instructions share SPECIAL3 opcode with other non-DSP
instructions, necessary logic was inserted for distinguishing
between instructions with SPECIAL3 opcode. For that purpose,
the instruction format for DSP instructions is added to
arch/mips/include/uapi/asm/inst.h.