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/am57xx_evm_defconfig b/configs/am57xx_evm_defconfig
index 0424887..6b11b34 100644
--- a/configs/am57xx_evm_defconfig
+++ b/configs/am57xx_evm_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SPL_SEPARATE_BSS=y
 CONFIG_SPL_DMA_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
+CONFIG_SPL_SPI_LOAD=y
 CONFIG_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x82000000
 CONFIG_FASTBOOT_BUF_SIZE=0x2F000000