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/spring_defconfig b/configs/spring_defconfig
index c008428..2bbfa6d 100644
--- a/configs/spring_defconfig
+++ b/configs/spring_defconfig
@@ -44,6 +44,8 @@
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_S5P=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_MODE=0
+CONFIG_SF_DEFAULT_SPEED=50000000
 CONFIG_SPI_FLASH_GIGADEVICE=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_SMC911X=y