em357: Corrected EM357 support including errata details

Original submitted code had only been tested with em358, but testing with
actual em357 revealed errors that this patch corrects.

Change-Id: I70cf31210de8ed84e3755a56e76261ad200322bb
Signed-off-by: Ed Beroset <beroset@ieee.org>
Reviewed-on: http://openocd.zylin.com/2581
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

tcl/target/em358.cfg

@@ -1,4 +1,3 @@
-#
 # Target configuration for the Silicon Labs EM358 chips
 
 #
@@ -9,6 +8,10 @@ if { ![info exists CHIPNAME] } {
    set CHIPNAME em358
 }
 
+if { ![info exists BSTAPID] } {
+  set BSTAPID 0x069aa62b
+}
+
 # 512K of flash in the em358 chips
 set FLASHSIZE 0x80000
 source [find target/em357.cfg]