Add default implementation of timer info

Add weak implementations of *_ticker_get_info which returns 1MHz and
a width of 32 bits. This allows the updated Ticker API to work with
existing devices.

Note - in the future when all targets have implemented
*_ticker_get_info these weak functions will be removed.

platform/mbed_retarget.cpp

@@ -27,6 +27,8 @@
 #include "platform/mbed_stats.h"
 #include "platform/mbed_critical.h"
 #include "platform/PlatformMutex.h"
+#include "us_ticker_api.h"
+#include "lp_ticker_api.h"
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>
@@ -1063,3 +1065,23 @@ extern "C" clock_t clock()
     _mutex->unlock();
     return t;
 }
+
+// temporary - Default to 1MHz at 32 bits if target does not have us_ticker_get_info
+MBED_WEAK const ticker_info_t* us_ticker_get_info()
+{
+    static const ticker_info_t info = {
+        1000000,
+        32
+    };
+    return &info;
+}
+
+// temporary - Default to 1MHz at 32 bits if target does not have lp_ticker_get_info
+MBED_WEAK const ticker_info_t* lp_ticker_get_info()
+{
+    static const ticker_info_t info = {
+        1000000,
+        32
+    };
+    return &info;
+}