RISC-V: Add a basic defconfig

This patch provides a basic defconfig for the RISC-V
architecture that enables enough kernel features to run a
basic Linux distribution on qemu's "virt" board for native
software development. Features include:

- serial console
- virtio block and network device support
- VFAT and ext2/3/4 filesystem support
- NFS client and NFS rootfs support
- an assortment of other kernel features required for
  running systemd

It also enables a number of drivers for physical hardware
that target the "SiFive U500" SoC and the corresponding
development platform.  These include:

- PCIe host controller support for the FPGA-based U500
  development platform (PCIE_XILINX)
- USB host controller support (OHCI/EHCI/XHCI)
- USB HID (keyboard/mouse) support
- USB mass storage support (bulk and UAS)
- SATA support (AHCI)
- ethernet drivers (MACB for a SoC-internal MAC block, microsemi
  ethernet phy, E1000E and R8169 for PCIe-connected external devices)
- DRM and framebuffer console support for PCIe-connected
  Radeon graphics chips

Signed-off-by: Karsten Merker <merker@debian.org>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>