NCS36510: Make RTC driver to operate on seconds instead of us.

In current implementation `rtc_read` function returns number of elapsed us and `rtc_write` function sets RTC time to specified value in us.
Mbed HAL API expects that these functions operate on seconds.
Since lp ticker is also based on RTC provide mechanism to trace elapsed seconds without modifying RTC registers.

Author: mprse

targets/TARGET_ONSEMI/TARGET_NCS36510/rtc_api.c

@@ -39,6 +39,15 @@
 #include "rtc.h"
 #include "cmsis_nvic.h"
 
+#define US_PER_SEC 1000000
+
+static time_t m_time_base;
+
+static uint32_t rtc_seconds_get()
+{
+    return (uint32_t)((fRtcRead() / US_PER_SEC) & 0xFFFFFFFF);
+}
+
 /* See rtc_apc.h for description */
 
 void rtc_init(void)
@@ -61,13 +70,20 @@ int rtc_isenabled(void)
 /* See rtc_apc.h for description */
 time_t rtc_read(void)
 {
-    return (uint32_t)(fRtcRead() & 0xFFFFFFFF); /* TODO Truncating 64 bit value to 32 bit */
+    return m_time_base + rtc_seconds_get();
 }
 
 /* See rtc_apc.h for description */
 void rtc_write(time_t t)
 {
-    fRtcWrite(t);
+    uint32_t seconds;
+    do {
+        seconds = rtc_seconds_get();
+        m_time_base = t - seconds;
+    /* If the number of seconds indicated by the counter changed during the
+       update of the time base, just repeat the update, now using the new
+       number of seconds. */
+    } while (seconds != rtc_seconds_get());
 }
 
 #endif /* DEVICE_RTC */