MIPS: Use dins to simplify __write_64bit_c0_split()

The code in __write_64bit_c0_split() is used by MIPS32 kernels running
on MIPS64 CPUs to write a 64-bit value to a 64-bit coprocessor 0
register using a single 64-bit dmtc0 instruction. It does this by
combining the 2x 32-bit registers used to hold the 64-bit value into a
single register, which in the existing code involves three steps:

  1) Zero extend register A which holds bits 31:0 of our data, since it
     may have previously held a sign-extended value.

  2) Shift register B which holds bits 63:32 of our data in bits 31:0
     left by 32 bits, such that the bits forming our data are in the
     position they'll be in the final 64-bit value & bits 31:0 of the
     register are zero.

  3) Or the two registers together to form the 64-bit value in one
     64-bit register.

From MIPS r2 onwards we have a dins instruction which can effectively
perform all 3 of those steps using a single instruction.

Add a path for MIPS r2 & beyond which uses dins to take bits 31:0 from
register B & insert them into bits 63:32 of register A, giving us our
full 64-bit value in register A with one instruction.

Since we know that MIPS r2 & above support the sel field for the dmtc0
instruction, we don't bother special casing sel==0. Omiting the sel
field would assemble to exactly the same instruction as when we
explicitly specify that it equals zero.

Signed-off-by: Paul Burton <paul.burton@mips.com>