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/at91sam9x5ek_mmc_defconfig b/configs/at91sam9x5ek_mmc_defconfig
index ff86f93..b615299 100644
--- a/configs/at91sam9x5ek_mmc_defconfig
+++ b/configs/at91sam9x5ek_mmc_defconfig
@@ -45,6 +45,7 @@
 CONFIG_NAND_ATMEL=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=30000000
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_DM_ETH=y
 CONFIG_MACB=y