board: sifive: add HiFive Unmatched board support

Add defconfig and board support for HiFive Unmatched.

Signed-off-by: Green Wan <green.wan@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>

configs/sifive_unmatched_defconfig

@@ -0,0 +1,54 @@
+CONFIG_RISCV=y
+CONFIG_SPL_GPIO_SUPPORT=y
+CONFIG_SYS_MALLOC_F_LEN=0x3000
+CONFIG_NR_DRAM_BANKS=1
+CONFIG_SPL_DM_SPI=y
+CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL=y
+CONFIG_SPL_SPI_SUPPORT=y
+CONFIG_DEFAULT_DEVICE_TREE="hifive-unmatched-a00"
+CONFIG_TARGET_SIFIVE_UNMATCHED=y
+CONFIG_ARCH_RV64I=y
+CONFIG_RISCV_SMODE=y
+CONFIG_DISTRO_DEFAULTS=y
+CONFIG_FIT=y
+CONFIG_SPL_LOAD_FIT_ADDRESS=0x84000000
+CONFIG_DISPLAY_CPUINFO=y
+CONFIG_DISPLAY_BOARDINFO=y
+CONFIG_DISPLAY_BOARDINFO_LATE=y
+CONFIG_SPL_SEPARATE_BSS=y
+CONFIG_SPL_DM_RESET=y
+CONFIG_SPL_YMODEM_SUPPORT=y
+CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_SPL_CLK=y
+CONFIG_DM_RESET=y
+CONFIG_CMD_PCI=y
+CONFIG_PCI=y
+CONFIG_DM_PCI=y
+CONFIG_PCI_PNP=y
+CONFIG_PCIE_DW_SIFIVE=y
+CONFIG_NVME=y
+CONFIG_DM_ETH=y
+CONFIG_NETDEVICES=y
+CONFIG_E1000=y
+CONFIG_USB=y
+CONFIG_CMD_USB=y
+CONFIG_DM_USB=y
+CONFIG_USB_STORAGE=y
+CONFIG_USB_XHCI_HCD=y
+CONFIG_USB_XHCI_PCI=y
+CONFIG_CMD_PART=y
+CONFIG_CMD_NVME=y
+CONFIG_SYS_USB_EVENT_POLL=y
+CONFIG_CMD_GPT=y
+CONFIG_CMD_GPT_RENAME=y
+CONFIG_CMD_EEPROM=y
+CONFIG_CMD_MEMINFO=y
+CONFIG_CMD_I2C=y
+CONFIG_DM_I2C=y
+CONFIG_SYS_I2C_OCORES=y
+CONFIG_CLK_SIFIVE_PRCI=y
+CONFIG_DM_PWM=y
+CONFIG_PWM_SIFIVE=y
+CONFIG_CMD_PWM=y
+CONFIG_SPL_USE_ARCH_MEMMOVE=n