Sansav2 : initializes SDRAM

The AS3525 SoC ships with an ARM PL172 MPMC controller
Also correct the memory sizes in tools/configure

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@18899 a1c6a512-1295-4272-9138-f99709370657

bootloader/sansa_as3525.c

@@ -36,6 +36,8 @@ void main(void)
     int i;
     unsigned char buf[8];
 
+    system_init();
+
     lcd_init_device();
     lcd_clear_display();