common: rename lcd_simplefb.c file to fdt_simplefb.c

Rename the file lcd_simplefb.c to fdt_simplefb.c to be aligned
with the configuration name and with the associated include file
./include/fdt_simplefb.h

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>