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/mx28evk_defconfig b/configs/mx28evk_defconfig
index be1feb3..187467d 100644
--- a/configs/mx28evk_defconfig
+++ b/configs/mx28evk_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_NAND_TRIMFFS=y
 CONFIG_CMD_SF=y
 CONFIG_CMD_SPI=y
+CONFIG_DEFAULT_SPI_BUS=2
 CONFIG_CMD_USB=y
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_MII=y