drivers: counter: update counter_mcux_rtc.c

Set LPO 1KHZ clock for RTC if clock source 'LPO' is selected.
The frdm_ke17z512 has no 32KHZ OSC, the RTC clock comes from SOSC,
RTC_CLKIN, LPO 1KHZ. But usually the SOSC is connected with 8MHZ
oscilator, so only 1kHZ LPO is usable.
Update the nxp,kinetis-rtc.yaml to select RTC clock source.

Signed-off-by: Anke Xiao <anke.xiao@nxp.com>