RISC-V changes for 4.15-rc8

This contains what I hope are the last RISC-V changes to go into 4.15.
I know it's a bit last minute, but I think they're all fairly small
changes:

* SR_* constants have been renamed to match the latest ISA
  specification.
* Some CONFIG_MMU #ifdef cruft has been removed.  We've never supported
  !CONFIG_MMU.
* __NR_riscv_flush_icache is now visible to userspace.  We were hoping
  to avoid making this public in order to force userspace to call the
  vDSO entry, but it looks like QEMU's user-mode emulation doesn't want
  to emulate a vDSO.  In order to allow glibc to fall back to a system
  call when the vDSO entry doesn't exist we're just
* Our defconfig is no long empty.  This is another one that just slipped
  through the cracks.  The defconfig isn't perfect, but it's at least
  close to what users will want for the first RISC-V development board.
  Getting closer is kind of splitting hairs here: none of the RISC-V
  specific drivers are in yet, so it's not like things will boot out of
  the box.

The only one that's strictly necessary is the __NR_riscv_flush_icache
change, as I want that to be part of the public API starting from our
first kernel so nobody has to worry about it.  The others are nice to
haves, but they seem sane for 4.15 to me.