Convert CONFIG_SPL_SPI_FLASH_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/cpu/armv8/zynqmp/Kconfig b/arch/arm/cpu/armv8/zynqmp/Kconfig
index f370970..6ad04cb 100644
--- a/arch/arm/cpu/armv8/zynqmp/Kconfig
+++ b/arch/arm/cpu/armv8/zynqmp/Kconfig
@@ -18,6 +18,9 @@
 config SPL_SERIAL_SUPPORT
 	default y
 
+config SPL_SPI_FLASH_SUPPORT
+	default y if ZYNQ_QSPI
+
 config SYS_BOARD
 	default "zynqmp"