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/km_kirkwood_defconfig b/configs/km_kirkwood_defconfig
index b47b9e3..b1a95cb 100644
--- a/configs/km_kirkwood_defconfig
+++ b/configs/km_kirkwood_defconfig
@@ -35,6 +35,7 @@
 CONFIG_BOOTCOUNT_BOOTLIMIT=3
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=8100000
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_MVGBE=y
 CONFIG_MII=y