lib: posix: clock: Add read of CLOCK_REALTIME
lib: posix: clock: Add read of CLOCK_REALTIME

Add a private variable `rt_clock_base` that can be used to determine a
real-time clock by using the `k_uptime_get` clock.  Once `clock_settime`
is added, this can allow us to have a meaningful real time clock.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

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