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/ficus-rk3399_defconfig b/configs/ficus-rk3399_defconfig
index 4e4e7dd..94c565e 100644
--- a/configs/ficus-rk3399_defconfig
+++ b/configs/ficus-rk3399_defconfig
@@ -44,6 +44,7 @@
 CONFIG_MMC_DW_ROCKCHIP=y
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_ROCKCHIP=y
+CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_RGMII=y