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/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index a14c5f2..b2faf1c 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -41,6 +41,8 @@
 CONFIG_SYS_BOOTCOUNT_ADDR=0x00900000
 CONFIG_FSL_ESDHC=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_MODE=0
+CONFIG_SF_DEFAULT_SPEED=25000000
 CONFIG_SPI_FLASH_GIGADEVICE=y
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_SPI_FLASH_SPANSION=y