asm: convert the ARM implementation of bit operations to the reference

The implementation of bit operations for ARM isn't actually
architecture-specific so it would rather be shared with
the upcoming port for AArch64 that won't provide optimized
implementation of bit operations.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
bitops: Add comment about generic bitops header usage

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>