[platform][sam3] on particular variants, start sram at 0x20700000

-on 96KB sam3s, the first sram bank of 64KB is repeated 8 times at 20000000.
the second bank starts at 20800000. On these versions if you start your
notion of sram at 20700000 then you get one contiguous 96KB of sram.

platform/sam3/rules.mk

@@ -12,6 +12,7 @@ DEFINES += \
 	__SAM3X8H__=1 \
 	SAM3XA=1
 MEMSIZE ?= 98304 
+MEMBASE := 0x20070000
 endif
 
 ifeq ($(MEMSIZE),)