common: rename functions lcd_dt_simplefb to fdt_simplefb

Rename the function named lcd_dt_simplefb* to fdt_simplefb* to be aligned
with the associated file name fdt_simplefb.h/fdt_simplefb.c

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>