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>

arch/riscv/configs/defconfig

+CONFIG_SMP=y
+CONFIG_PCI=y
+CONFIG_PCIE_XILINX=y
+CONFIG_SYSVIPC=y
+CONFIG_POSIX_MQUEUE=y
+CONFIG_IKCONFIG=y
+CONFIG_IKCONFIG_PROC=y
+CONFIG_CGROUPS=y
+CONFIG_CGROUP_SCHED=y
+CONFIG_CFS_BANDWIDTH=y
+CONFIG_CGROUP_BPF=y
+CONFIG_NAMESPACES=y
+CONFIG_USER_NS=y
+CONFIG_BLK_DEV_INITRD=y
+CONFIG_EXPERT=y
+CONFIG_CHECKPOINT_RESTORE=y
+CONFIG_BPF_SYSCALL=y
+CONFIG_NET=y
+CONFIG_PACKET=y
+CONFIG_UNIX=y
+CONFIG_INET=y
+CONFIG_IP_MULTICAST=y
+CONFIG_IP_ADVANCED_ROUTER=y
+CONFIG_IP_PNP=y
+CONFIG_IP_PNP_DHCP=y
+CONFIG_IP_PNP_BOOTP=y
+CONFIG_IP_PNP_RARP=y
+CONFIG_NETLINK_DIAG=y
+CONFIG_DEVTMPFS=y
+CONFIG_BLK_DEV_LOOP=y
+CONFIG_VIRTIO_BLK=y
+CONFIG_BLK_DEV_SD=y
+CONFIG_BLK_DEV_SR=y
+CONFIG_ATA=y
+CONFIG_SATA_AHCI=y
+CONFIG_SATA_AHCI_PLATFORM=y
+CONFIG_NETDEVICES=y
+CONFIG_VIRTIO_NET=y
+CONFIG_MACB=y
+CONFIG_E1000E=y
+CONFIG_R8169=y
+CONFIG_MICROSEMI_PHY=y
+CONFIG_INPUT_MOUSEDEV=y
+CONFIG_SERIAL_8250=y
+CONFIG_SERIAL_8250_CONSOLE=y
+CONFIG_SERIAL_OF_PLATFORM=y
+# CONFIG_PTP_1588_CLOCK is not set
+CONFIG_DRM=y
+CONFIG_DRM_RADEON=y
+CONFIG_FRAMEBUFFER_CONSOLE=y
+CONFIG_USB=y
+CONFIG_USB_XHCI_HCD=y
+CONFIG_USB_XHCI_PLATFORM=y
+CONFIG_USB_EHCI_HCD=y
+CONFIG_USB_EHCI_HCD_PLATFORM=y
+CONFIG_USB_OHCI_HCD=y
+CONFIG_USB_OHCI_HCD_PLATFORM=y
+CONFIG_USB_STORAGE=y
+CONFIG_USB_UAS=y
+CONFIG_VIRTIO_MMIO=y
+CONFIG_RAS=y
+CONFIG_EXT4_FS=y
+CONFIG_EXT4_FS_POSIX_ACL=y
+CONFIG_AUTOFS4_FS=y
+CONFIG_MSDOS_FS=y
+CONFIG_VFAT_FS=y
+CONFIG_TMPFS=y
+CONFIG_TMPFS_POSIX_ACL=y
+CONFIG_NFS_FS=y
+CONFIG_NFS_V4=y
+CONFIG_NFS_V4_1=y
+CONFIG_NFS_V4_2=y
+CONFIG_ROOT_NFS=y
+# CONFIG_RCU_TRACE is not set
+CONFIG_CRYPTO_USER_API_HASH=y