ext: simplelink: Use monotonic time

[pavlohamov]

Rationale: In case if

clock_gettime(CLOCK_REALTIME, &abstime);

derives RTC time system will hang on bootup

Zephyr sem_timedwait uses uptime, thus user must use motonic time instead of realtime.

[pavlohamov]

@nashif Not sure who is responsible for this repo. Could you be so kind to take a look?

[cfriedt]

@pavlohamov - LGTM. Do you already have a PR to Zephyr's main repository to run this change through CI? If not, can you create one that will exercise this code path? Please prefix it with [DNM] and / or apply the DNM label.

[cfriedt]

@pavlohamov - normally, it's necessary to create a DNM pull request to zephyrproject-rtos/zephyr in order to verify that a HAL change does not introduce any regressions in the main build. Can you please create one?

https://docs.zephyrproject.org/latest/develop/modules.html#process-for-submitting-changes-to-existing-modules

[cfriedt]

@vaishnavachath - I've asked the author to provide the required DNM PR to verify that this works, but there has been no response. I'll leave it up to you to test / merge if you feel it's appropriate.