Merge remote-tracking branch 'remotes/palmer/tags/riscv-for-master-3.2-part2' into staging

RISC-V Updates for 3.2, Part 2

This patch set contains a handful of Michael's CSR-related cleanups,
which should allow us to proceed with more outstanding bug fixes that
depend on them.

Additionally, there is a patch that turns on USB.  This works for me
when the kernel has the appropriate drivers (which will soon be in
defconfig) and I pass

    -device usb-ehci
    -drive id=my_usb_disk,file=usbdisk.img,if=none,format=raw
    -device usb-storage,drive=my_usb_disk

to QEMU.

# gpg: Signature made Fri 11 Jan 2019 18:05:02 GMT
# 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/palmer/tags/riscv-for-master-3.2-part2:
  default-configs: Enable USB support for RISC-V machines
  RISC-V: Implement existential predicates for CSRs
  RISC-V: Implement atomic mip/sip CSR updates
  RISC-V: Implement modular CSR helper interface

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