Merge remote-tracking branch 'remotes/alistair/tags/pull-riscv-to-apply-20210511' into staging

A large collection of RISC-V fixes, improvements and features

 - Clenaup some left over v1.9 code
 - Documentation improvements
 - Support for the shakti_c machine
 - Internal cleanup of the CSR accesses
 - Updates to the OpenTitan platform
 - Support for the virtio-vga
 - Fix for the saturate subtract in vector extensions
 - Experimental support for the ePMP spec
 - A range of other internal code cleanups and bug fixes

# gpg: Signature made Tue 11 May 2021 11:17:10 BST
# 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-20210511: (42 commits)
  target/riscv: Fix the RV64H decode comment
  target/riscv: Consolidate RV32/64 16-bit instructions
  target/riscv: Consolidate RV32/64 32-bit instructions
  target/riscv: Remove an unused CASE_OP_32_64 macro
  target/riscv: Remove the unused HSTATUS_WPRI macro
  target/riscv: Remove the hardcoded SATP_MODE macro
  target/riscv: Remove the hardcoded MSTATUS_SD macro
  target/riscv: Remove the hardcoded HGATP_MODE macro
  target/riscv: Remove the hardcoded SSTATUS_SD macro
  target/riscv: Remove the hardcoded RVXLEN macro
  target/riscv: fix a typo with interrupt names
  fpu/softfloat: set invalid excp flag for RISC-V muladd instructions
  hw/riscv: Fix OT IBEX reset vector
  target/riscv: fix exception index on instruction access fault
  target/riscv: fix vrgather macro index variable type bug
  target/riscv: Add ePMP support for the Ibex CPU
  target/riscv/pmp: Remove outdated comment
  target/riscv: Add a config option for ePMP
  target/riscv: Implementation of enhanced PMP (ePMP)
  target/riscv: Add ePMP CSR access functions
  ...

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