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/am43xx_evm_qspiboot_defconfig b/configs/am43xx_evm_qspiboot_defconfig
index 0d7bd72..b644273 100644
--- a/configs/am43xx_evm_qspiboot_defconfig
+++ b/configs/am43xx_evm_qspiboot_defconfig
@@ -39,6 +39,7 @@
 CONFIG_DM_MMC=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=48000000
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_MII=y
 CONFIG_DRIVER_TI_CPSW=y