riscv: Support booting SiFive Unmatched from SPI.

Configure SPI flash devices into SPL.  Add SPI boot option to spl.c.
Document how to format flash for booting.

Signed-off-by: Thomas Skibo <thomas-git@skibo.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/configs/sifive_unmatched_defconfig b/configs/sifive_unmatched_defconfig
index 9cc18b0..d400ed0 100644
--- a/configs/sifive_unmatched_defconfig
+++ b/configs/sifive_unmatched_defconfig
@@ -8,6 +8,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="hifive-unmatched-a00"
 CONFIG_SPL_MMC=y
 CONFIG_SPL=y
+CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI=y
 CONFIG_AHCI=y
 CONFIG_TARGET_SIFIVE_UNMATCHED=y
@@ -23,17 +24,22 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_ID_EEPROM=y
 CONFIG_SPL_SEPARATE_BSS=y
+CONFIG_SPL_DM_SPI_FLASH=y
 CONFIG_SPL_DM_RESET=y
+CONFIG_SPL_SPI_LOAD=y
 CONFIG_CMD_EEPROM=y
 CONFIG_CMD_MEMINFO=y
 CONFIG_CMD_PWM=y
 CONFIG_CMD_GPT_RENAME=y
 CONFIG_CMD_PCI=y
 CONFIG_CMD_USB=y
+CONFIG_USE_ENV_SPI_BUS=y
+CONFIG_ENV_SPI_BUS=1
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SCSI_AHCI=y
 CONFIG_AHCI_PCI=y
 CONFIG_SPL_CLK=y
+CONFIG_SPI_FLASH_ISSI=y
 CONFIG_SYS_I2C_EEPROM_ADDR=0x54
 CONFIG_E1000=y
 CONFIG_NVME=y