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/at91sam9n12ek_nandflash_defconfig b/configs/at91sam9n12ek_nandflash_defconfig
index fd83ba0..b04d95b 100644
--- a/configs/at91sam9n12ek_nandflash_defconfig
+++ b/configs/at91sam9n12ek_nandflash_defconfig
@@ -43,6 +43,7 @@
 CONFIG_SPL_GENERATE_ATMEL_PMECC_HEADER=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=30000000
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_AT91=y