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/popmetal-rk3288_defconfig b/configs/popmetal-rk3288_defconfig
index ca194fd..ac50729 100644
--- a/configs/popmetal-rk3288_defconfig
+++ b/configs/popmetal-rk3288_defconfig
@@ -52,6 +52,7 @@
 CONFIG_MMC_DW=y
 CONFIG_MMC_DW_ROCKCHIP=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_GMAC_ROCKCHIP=y