RISC-V: kconfig cleanups

These three kconfig cleanups were found by ulfalyzer.  They're all
things we were selecting that were undefined, either because they'd been
remove upstream or are part of a future RISC-V submission.

* ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE is obselete.
* RISCV_IRQ_INTC is the old name for our interrupt controller driver,
  it'll be changed for the final submission and doesn't exist now.
* ARCH_WANT_OPTIONAL_GPIOLIB is obselete.