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/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index 530ff79..de1016e 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -18,6 +18,7 @@
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_SPI_LOAD=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_UNZIP=y
 CONFIG_CMD_DFU=y