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/pxm2_defconfig b/configs/pxm2_defconfig
index 65e13a6..b8b1f13 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -25,6 +25,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_SPI_LOAD=y
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot# "