spl: spi: Move CONFIG_SPL_SPI_LOAD to Kconfig

Add Kconfig entry for CONFIG_SPL_SPI_LOAD symbol and move all
configurations using it to Kconfig.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 884cc8c..5926004 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -21,6 +21,7 @@
 CONFIG_BOOTCOMMAND="run findfdt; run distro_bootcmd; run legacy_bootcmd"
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x80
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_PROMPT="CM-FX6 # "
 # CONFIG_CMD_XIMG is not set
 CONFIG_CMD_GREPENV=y