Merge remote-tracking branch 'remotes/alistair/tags/pull-riscv-to-apply-20201109...

This fixes two bugs in the RISC-V port. One is a bug in the
Ibex PLIC, the other fixes the Hypvervisor access functions.

# gpg: Signature made Tue 10 Nov 2020 03:53:49 GMT
# gpg:                using RSA key F6C4AC46D4934868D3B8CE8F21E10D29DF977054
# gpg: Good signature from "Alistair Francis <alistair@alistair23.me>" [full]
# Primary key fingerprint: F6C4 AC46 D493 4868 D3B8  CE8F 21E1 0D29 DF97 7054

* remotes/alistair/tags/pull-riscv-to-apply-20201109:
  hw/intc/ibex_plic: Clear the claim register when read
  target/riscv: Split the Hypervisor execute load helpers
  target/riscv: Remove the hyp load and store functions
  target/riscv: Remove the HS_TWO_STAGE flag
  target/riscv: Set the virtualised MMU mode when doing hyp accesses
  target/riscv: Add a virtualised MMU Mode

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