Convert CONFIG_DEFAULT_SPI_* to Kconfig

This converts the following to Kconfig:
   CONFIG_DEFAULT_SPI_BUS
   CONFIG_DEFAULT_SPI_MODE

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 4bcc5c4..0b07b3b 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1040,10 +1040,20 @@
 	  everything is working properly.
 
 config CMD_SPI
-	bool "sspi"
+	bool "sspi - Command to access spi device"
 	help
 	  SPI utility command.
 
+config DEFAULT_SPI_BUS
+	int "default spi bus used by sspi command"
+	depends on CMD_SPI
+	default 0
+
+config DEFAULT_SPI_MODE
+	hex "default spi mode used by sspi command (see include/spi.h)"
+	depends on CMD_SPI
+	default 0
+
 config CMD_TSI148
 	bool "tsi148 - Command to access tsi148 device"
 	help