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/configs/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index c6dcb45..2267818 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -13,6 +13,8 @@
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_SPI=y
+CONFIG_DEFAULT_SPI_BUS=1
+CONFIG_DEFAULT_SPI_MODE=4
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y