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/k2g_evm_defconfig b/configs/k2g_evm_defconfig
index 8a2dc7f..87b887c 100644
--- a/configs/k2g_evm_defconfig
+++ b/configs/k2g_evm_defconfig
@@ -18,6 +18,7 @@
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
+CONFIG_SPL_SPI_LOAD=y
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_GPIO is not set
 # CONFIG_CMD_GPT is not set