lib/bitrev: Introduce GENERIC_BITREVERSE

The generic bit reversal implementation is controlled by
!HAVE_ARCH_BITREVERSE. This makes it difficult for architectures to
provide a hardware-accelerated implementation while being able to
fall back to the generic version if needed.

This patch adds GENERIC_BITREVERSE, so bitreverse API is controlled by
BITREVERSE, GENERIC_BITREVERSE and HAVE_ARCH_BITREVERSE options. The
relationship between them is described as follows:

 - BITREVERSE is selected by user code; it's required to generate the API;
 - Architectures may select HAVE_ARCH_BITREVERSE and provide an arch
   implementation in arch/$(ARCH)/include/asm/bitrev.h.
 - if HAVE_ARCH_BITREVERSE isn't set, BITREVERSE selects GENERIC_BITREVERSE;
 - if GENERIC_BITREVERSE is set and HAVE_ARCH_BITREVERSE is not, the kernel
   provides generic implementation only, and wires bitrevXX() to it.
 - if HAVE_ARCH_BITREVERSE is set and GENERIC_BITREVERSE is not, the arch
   code provides __arch_bitrevXX(), and it is wired to bitrevXX();
 - if both GENERIC_BITREVERSE and HAVE_ARCH_BITREVERSE are selected, the kernel
   generates generic___bitrev(), but wires bitrev() to the __arch_bitrev().

The last option allows architectures to use generic___bitrev() as a
fallback option.

Drivers and core code should never select GENERIC_BITREVERSE or
HAVE_ARCH_BITREVERSE explicitly.

Architectures that require generic bitreverse API as a fallback should
explicitly enable GENERIC_BITREVERSE together with HAVE_ARCH_BITREVERSE.

Signed-off-by: Yury Norov <ynorov@nvidia.com>

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