flash: add stm32l Revision X support

Revision X is not mentioned in the latest RM0038 rev5, however it has been
confirmed correct by ST using ST-LINK Utilty.

Change-Id: I65210e512ea25818a1d0d3b223502ebd7535b29d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/742
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/flash/nor/stm32lx.c

@@ -618,6 +618,10 @@ static int stm32lx_get_info(struct flash_bank *bank, char *buf, int buf_size)
 				snprintf(buf, buf_size, "Y");
 				break;
 
+			case 0x1018:
+				snprintf(buf, buf_size, "X");
+				break;
+
 			case 0x1038:
 				snprintf(buf, buf_size, "W");
 				break;