Merge remote-tracking branch 'remotes/riscv/tags/riscv-for-master-3.1-sf0' into staging

First RISC-V Patch Set for the 3.1 Soft Freeze

This pull request contains a handful of patches that have been floating
around various trees for a while but haven't made it upstream.  These
patches all appear quite safe.  They're all somewhat independent from
each other:

* One refactors our IRQ management function to allow multiple interrupts
  to be raised an once.  This patch has no functional difference.
* Cleaning up the op_helper/cpu_helper split.  This patch has no
  functional difference.
* Updates to various constants to keep them in sync with the latest ISA
  specification and to remove some non-standard bits that snuck in.
* A fix for a memory leak in the PLIC driver.
* A fix to our device tree handling to avoid provinging a NULL string.

I've given this my standard test: building the port, booting a Fedora
root filesytem on the latest Linux tag, and then shutting down that
image.  Essentially I'm just following the QEMU RISC-V wiki page's
instructions.  Everything looks fine here.

We have a lot more outstanding patches so I'll definately be submitting
another PR for the soft freeze.

# gpg: Signature made Wed 17 Oct 2018 21:17:52 BST
# gpg:                using RSA key EF4CA1502CCBAB41
# gpg: Good signature from "Palmer Dabbelt <palmer@dabbelt.com>"
# gpg:                 aka "Palmer Dabbelt <palmer@sifive.com>"
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 00CE 76D1 8349 60DF CE88  6DF8 EF4C A150 2CCB AB41

* remotes/riscv/tags/riscv-for-master-3.1-sf0:
  RISC-V: Don't add NULL bootargs to device-tree
  RISC-V: Add missing free for plic_hart_config
  RISC-V: Update CSR and interrupt definitions
  RISC-V: Move non-ops from op_helper to cpu_helper
  RISC-V: Allow setting and clearing multiple irqs

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>