cpu/atmega1284p: configure RAM and ROM length

Info taken from https://www.microchip.com/wwwproducts/en/atmega1284p

TODO: update "board not enough memory".

cpu/atmega1284p/Makefile.include

@@ -4,5 +4,8 @@ USEMODULE += atmega_common
 # define path to atmega common module, which is needed for this CPU
 export ATMEGA_COMMON = $(RIOTCPU)/atmega_common/
 
+RAM_LEN = 16K
+ROM_LEN = 128K
+
 # CPU depends on the atmega common module, so include it
 include $(ATMEGA_COMMON)Makefile.include