lib: posix: clock: Use k_uptime_get() to compute tv_nsec
lib: posix: clock: Use k_uptime_get() to compute tv_nsec

Use k_uptime_get() to compute both tv_sec and tv_nsec members
of timespec structure.

Fixes #8009

Signed-off-by: Ramakrishna Pallala <8cb5df094165bcc31e7d772ccd8d87d8c4d8f761@intel.com>

galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,mbolivar/zephyr,mbolivar/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyriot/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,kraj/zephyr,punitvara/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,kraj/zephyr,explora26/zephyr,finikorg/zephyr,kraj/zephyr,zephyriot/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,mbolivar/zephyr,nashif/zephyr,zephyriot/zephyr