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/birdland_bav335a_defconfig b/configs/birdland_bav335a_defconfig
index 17015b9..eea1223 100644
--- a/configs/birdland_bav335a_defconfig
+++ b/configs/birdland_bav335a_defconfig
@@ -48,6 +48,7 @@
 CONFIG_FASTBOOT_BUF_ADDR=0x82000000
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=24000000
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_MII=y
 CONFIG_DRIVER_TI_CPSW=y