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/theadorable_debug_defconfig b/configs/theadorable_debug_defconfig
index a7d02e9..cd52220 100644
--- a/configs/theadorable_debug_defconfig
+++ b/configs/theadorable_debug_defconfig
@@ -55,6 +55,7 @@
 CONFIG_DM_GPIO=y
 # CONFIG_MMC is not set
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=27777777
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHY_MARVELL=y