arch/riscv: Add bitrev.h file to support rev8 and brev8

The RISC-V Bit-manipulation Extension for Cryptography (Zbkb) provides
the 'brev8' instruction, which reverses the bits within each byte.
Combined with the 'rev8' instruction (from Zbb or Zbkb), which reverses
the byte order of a register, we can efficiently implement 16-bit,
32-bit, and (on RV64) 64-bit bit reversal.

This is significantly faster than the default software table-lookup
implementation in lib/bitrev.c, as it replaces memory accesses and
multiple arithmetic operations with just two or three hardware
instructions.

Select HAVE_ARCH_BITREVERSE as well as GENERIC_BITREVERSE,
and provide <asm/bitrev.h> to utilize these instructions when
the Zbkb extension is available at runtime via the alternatives
mechanism.

[Yury: select the options conditionally on BITREVERSE]

Link: https://docs.riscv.org/reference/isa/unpriv/b-st-ext.html
Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Yury Norov <ynorov@nvidia.com>

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