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/puma-rk3399_defconfig b/configs/puma-rk3399_defconfig
index 3fcff25..3547ec6 100644
--- a/configs/puma-rk3399_defconfig
+++ b/configs/puma-rk3399_defconfig
@@ -67,6 +67,7 @@
 CONFIG_MMC_SDHCI_SDMA=y
 CONFIG_MMC_SDHCI_ROCKCHIP=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y