SysTimer should be buildable without lpticker

Tickless from us ticker is possible since

    b32b996 Use us ticker for tickless on devs with wrapper

therefore tickless's SysTimer should be buildable even without
lpticker enabled.

Author: LDong-Arm

rtos/TARGET_CORTEX/SysTimer.cpp

@@ -21,9 +21,8 @@
  */
 #include "rtos/TARGET_CORTEX/SysTimer.h"
 
-#if DEVICE_LPTICKER
+#if MBED_TICKLESS
 
-#include "hal/lp_ticker_api.h"
 #include "mbed_critical.h"
 #include "mbed_assert.h"
 #if defined(TARGET_CORTEX_A)
@@ -57,14 +56,6 @@ extern "C" IRQn_ID_t mbed_get_a9_tick_irqn(void);
 namespace rtos {
 namespace internal {
 
-SysTimer::SysTimer() :
-    TimerEvent(get_lp_ticker_data()), _time_us(0), _tick(0)
-{
-    _time_us = ticker_read_us(_ticker_data);
-    _suspend_time_passed = true;
-    _suspended = false;
-}
-
 SysTimer::SysTimer(const ticker_data_t *data) :
     TimerEvent(data), _time_us(0), _tick(0)
 {

rtos/TARGET_CORTEX/SysTimer.h

@@ -22,7 +22,7 @@
 #ifndef MBED_SYS_TIMER_H
 #define MBED_SYS_TIMER_H
 
-#if DEVICE_LPTICKER || defined(DOXYGEN_ONLY)
+#if MBED_TICKLESS || defined(DOXYGEN_ONLY)
 
 #include "platform/NonCopyable.h"
 #include "drivers/TimerEvent.h"
@@ -48,7 +48,6 @@ namespace internal {
 class SysTimer: private mbed::TimerEvent, private mbed::NonCopyable<SysTimer> {
 public:
 
-    SysTimer();
     SysTimer(const ticker_data_t *data);
     virtual ~SysTimer();
 
@@ -119,8 +118,8 @@ class SysTimer: private mbed::TimerEvent, private mbed::NonCopyable<SysTimer> {
     static void _set_irq_pending();
     us_timestamp_t _time_us;
     uint64_t _tick;
-    bool _suspend_time_passed;
-    bool _suspended;
+    bool _suspend_time_passed = true;
+    bool _suspended = false;
 };
 
 /**