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/k2e_hs_evm_defconfig b/configs/k2e_hs_evm_defconfig
index 67f6fb1..11c9fda 100644
--- a/configs/k2e_hs_evm_defconfig
+++ b/configs/k2e_hs_evm_defconfig
@@ -36,6 +36,7 @@
 CONFIG_NAND_DAVINCI=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=30000000
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MARVELL=y