Convert CONFIG_SF_DEFAULT_* to Kconfig

This converts the following to Kconfig:
  CONFIG_SF_DEFAULT_BUS
  CONFIG_SF_DEFAULT_CS
  CONFIG_SF_DEFAULT_MODE
  CONFIG_SF_DEFAULT_SPEED

I use moveconfig script and then manual check on generated u-boot.cfg
to solve the remaining issue.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index c8d1e83..6156ac2 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -57,6 +57,8 @@
 CONFIG_NAND=y
 CONFIG_NAND_MXS=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_MODE=0
+CONFIG_SF_DEFAULT_SPEED=25000000
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_SPI_FLASH_EON=y
 CONFIG_SPI_FLASH_GIGADEVICE=y