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/stmark2_defconfig b/configs/stmark2_defconfig
index 48e851d..d74ba67 100644
--- a/configs/stmark2_defconfig
+++ b/configs/stmark2_defconfig
@@ -23,6 +23,7 @@
 # CONFIG_NET is not set
 CONFIG_MTD_DEVICE=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=50000000
 CONFIG_SPI_FLASH_ISSI=y
 CONFIG_SPI_FLASH_MTD=y
 CONFIG_SPI=y