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-am57x_defconfig b/configs/cl-som-am57x_defconfig
index cd780cd..fd8353e 100644
--- a/configs/cl-som-am57x_defconfig
+++ b/configs/cl-som-am57x_defconfig
@@ -37,6 +37,7 @@
 CONFIG_LED_STATUS_STATE=2
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=48000000
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_SPI_FLASH_EON=y
 CONFIG_SPI_FLASH_GIGADEVICE=y