stm32f7: enable existing ltdc

This uses the existing f4 code as a new shared common base code.

lib/stm32/f4/Makefile

@@ -54,7 +54,7 @@ OBJS		+= gpio_common_all.o gpio_common_f0234.o gpio.o
 OBJS		+= hash_common_f24.o
 OBJS		+= i2c_common_v1.o
 OBJS		+= iwdg_common_all.o
-OBJS		+= ltdc.o 
+OBJS		+= ltdc_common_f47.o
 OBJS		+= pwr_common_v1.o pwr.o
 OBJS		+= rcc_common_all.o rcc.o
 OBJS		+= rng_common_v1.o