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/nitrogen6s_defconfig b/configs/nitrogen6s_defconfig
index 40c09d7..5bf2b3a 100644
--- a/configs/nitrogen6s_defconfig
+++ b/configs/nitrogen6s_defconfig
@@ -38,6 +38,8 @@
 CONFIG_FASTBOOT_BUF_ADDR=0x12000000
 CONFIG_FSL_ESDHC=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_MODE=0
+CONFIG_SF_DEFAULT_SPEED=25000000
 CONFIG_SPI_FLASH_SST=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y