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/am335x_evm_spiboot_defconfig b/configs/am335x_evm_spiboot_defconfig
index 874d67d..23ad679 100644
--- a/configs/am335x_evm_spiboot_defconfig
+++ b/configs/am335x_evm_spiboot_defconfig
@@ -14,6 +14,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
+CONFIG_SPL_SPI_LOAD=y
 CONFIG_FASTBOOT=y
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set