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/am335x_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index f455b01..6afda72 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -39,6 +39,7 @@
 CONFIG_DM_MMC=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=24000000
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_DM_ETH=y
 CONFIG_PHY_GIGE=y