driver: clock: npcx: remove the guard for npcx_clock_get_sleep_ticks

The function npcx_clock_get_sleep_ticks is currently guarded by
CONFIG_PM && CONFIG_NPCX_PM_TRACE. The other codes guarded by
CONFIG_NPCX_PM_TRACE is used to trace and will print a lot of messages.
The user who wants to use npcx_clock_get_sleep_ticks has to enable this
flag and get a lot of console spam. This commit removes the guard
CONFIG_NPCX_PM_TRACE and makes this function is available when
CONFIG_PM is defined.

Signed-off-by: Jun Lin <CHLin56@nuvoton.com>