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/socfpga_sr1500_defconfig b/configs/socfpga_sr1500_defconfig
index d984047..b8de47a 100644
--- a/configs/socfpga_sr1500_defconfig
+++ b/configs/socfpga_sr1500_defconfig
@@ -53,6 +53,7 @@
 CONFIG_MMC_DW=y
 CONFIG_MTD_DEVICE=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=100000000
 CONFIG_SPI_FLASH_STMICRO=y
 # CONFIG_SPI_FLASH_USE_4K_SECTORS is not set
 CONFIG_PHY_MARVELL=y