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/mx6ull_14x14_evk_plugin_defconfig b/configs/mx6ull_14x14_evk_plugin_defconfig
index 831b8cc..990ea71 100644
--- a/configs/mx6ull_14x14_evk_plugin_defconfig
+++ b/configs/mx6ull_14x14_evk_plugin_defconfig
@@ -34,6 +34,8 @@
 CONFIG_FSL_ESDHC=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_MODE=0
+CONFIG_SF_DEFAULT_SPEED=40000000
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y