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/rock_defconfig b/configs/rock_defconfig
index 2de51e8..19c3858 100644
--- a/configs/rock_defconfig
+++ b/configs/rock_defconfig
@@ -41,6 +41,7 @@
 CONFIG_MMC_DW=y
 CONFIG_MMC_DW_ROCKCHIP=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_PINCTRL=y
 CONFIG_DM_PMIC=y
 # CONFIG_SPL_PMIC_CHILDREN is not set