aspeed: Set ast2600 evb spi flash size to 64 MiB

AST2600 evb has 64 MiB flash by default, so machine config could
provide a default value for FLASH_SIZE.

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Change-Id: Idca8734f950a16966a5a7322c0c50d6e63d9106e
diff --git a/meta-aspeed/conf/machine/evb-ast2600.conf b/meta-aspeed/conf/machine/evb-ast2600.conf
index 9fb38e0..e011045 100644
--- a/meta-aspeed/conf/machine/evb-ast2600.conf
+++ b/meta-aspeed/conf/machine/evb-ast2600.conf
@@ -5,3 +5,6 @@
 require conf/machine/include/ast2600.inc
 
 SERIAL_CONSOLES = "115200;ttyS4"
+
+# AST2600 EVB has 64MB SPI flash
+FLASH_SIZE = "65536"