Fix for issue #8155 (NRF52832: time stops after 35 minutes)
Fix for issue #8155 (NRF52832: time stops after 35 minutes)

Low power Timer is used as RTC for platforms that don't have HW RTC capabilities (like NRF52832).
`_rtc_lpticker_read(void)` function currently uses `Timer::read()` function to trace elapsed time.
`Timer::read()` returns seconds represented as `float` value, but this value is calculated from `int` since `Timer::read_us()` returns `int`.
This limits time tracing to ~35 min.
To fix this problem we will use `timer::read_high_resolution_us()` (which returns unsigned 64 bit value) instead of `Timer::read()`.

mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed