stm32l1: syscfg: add USB pullup control definition

On original stm32l1s, this internal pullup was out of spec, and not
recommended for use.  But the -A parts have this fixed, so make sure we
can use it.

include/libopencm3/stm32/l1/syscfg.h

@@ -38,4 +38,7 @@
 
 #include <libopencm3/stm32/common/syscfg_common_l1f234.h>
 
+/** Enable internal USB pullup iff USB periph is not powered down */
+#define SYSCFG_PMC_USB_PU			(1<<0)
+
 #endif