Added 512K flashing support for em3587

The Silicon Labs EM3587 and EM3588 may have 512K of flash.
This fix allows for 512K to be specifiied on the command line
when flashing a device.

Change-Id: I18cc4bd0d14e1f2069066734a7396bcccf3de941
Signed-off-by: Byron Kubert <byronk@google.com>
Reviewed-on: http://openocd.zylin.com/3795
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/flash/nor/em357.c

@@ -702,6 +702,11 @@ static int em357_probe(struct flash_bank *bank)
 			num_pages = 128;
 			page_size = 2048;
 			break;
+		case 0x80000:
+			/* 512k -- 256 2k pages */
+			num_pages = 256;
+			page_size = 2048;
+			break;
 		default:
 			LOG_WARNING("No size specified for em357 flash driver, assuming 192k!");
 			num_pages = 96;