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/cl-som-imx7_defconfig b/configs/cl-som-imx7_defconfig
index 3501e2b..2b26e66 100644
--- a/configs/cl-som-imx7_defconfig
+++ b/configs/cl-som-imx7_defconfig
@@ -51,6 +51,8 @@
 CONFIG_DM_MMC=y
 CONFIG_FSL_ESDHC=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_MODE=0
+CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_SPI_FLASH_EON=y
 CONFIG_SPI_FLASH_GIGADEVICE=y