ARM: mvebu: set correct mem_size for db-88f6820-amc

The db-88f6820-amc has four chips with 2Gb density giving a total of 1GB
DRAM. Update the board_topology_map to reflect the correct
configuration.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/Marvell/db-88f6820-amc/db-88f6820-amc.c b/board/Marvell/db-88f6820-amc/db-88f6820-amc.c
index 40fa599..ac58f90 100644
--- a/board/Marvell/db-88f6820-amc/db-88f6820-amc.c
+++ b/board/Marvell/db-88f6820-amc/db-88f6820-amc.c
@@ -66,7 +66,7 @@
 	      {0x1, 0, 0, 0} },
 	    SPEED_BIN_DDR_1866L,	/* speed_bin */
 	    BUS_WIDTH_8,		/* memory_width */
-	    MEM_4G,			/* mem_size */
+	    MEM_2G,			/* mem_size */
 	    DDR_FREQ_800,		/* frequency */
 	    0, 0,			/* cas_l cas_wl */
 	    HWS_TEMP_LOW,		/* temperature */