Convert CONFIG_LCD_DT_SIMPLEFB to Kconfig

This converts the following to Kconfig:
   CONFIG_LCD_DT_SIMPLEFB

This patch also renames this config to CONFIG_FDT_SIMPLEFB as the code in
common/lcd_simplefb.c support CONFIG_LCD and CONFIG_VIDEO.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
diff --git a/configs/rpi_defconfig b/configs/rpi_defconfig
index 8acf04d..0baef3b 100644
--- a/configs/rpi_defconfig
+++ b/configs/rpi_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_MISC_INIT_R=y
+CONFIG_FDT_SIMPLEFB=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_MMC=y