micropython only requires 512kB of BRAM

Mikey points out that our stack grows down from 512kB and our
heap is below that too, so we can reduce our BRAM requirements,
which allowing some smaller FPGA boards to work. Not sure why
I thought we were using memory between 512kB and 1MB.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

fpga/toplevel.vhd

@@ -13,7 +13,7 @@ use work.wishbone_types.all;
 -- 0xc0002000: UART0 (for host communication)
 entity toplevel is
   generic (
-    MEMORY_SIZE   : positive := 1048576;
+    MEMORY_SIZE   : positive := 524288;
     RAM_INIT_FILE : string   := "firmware.hex");
 	port(
 		clk       : in  std_logic;