cpu/atmega256rfr2: configure RAM and ROM length

Info taken from https://www.microchip.com/wwwproducts/en/atmega256rfr2

TODO: update "board not enough memory".

cpu/atmega256rfr2/Makefile.include

@@ -10,5 +10,8 @@ export UNDEF += $(BINDIR)/cpu/startup.o
 #include periph module
 USEMODULE += periph
 
+RAM_LEN = 32K
+ROM_LEN = 256K
+
 # CPU depends on the atmega common module, so include it
 include $(ATMEGA_COMMON)Makefile.include