drivers: timer: mcux: lptmr: add dependency on CONFIG_PM

The introduction of cc2c05a9 caused
CONFIG_MCUX_LPTMR_TIMER to always be enabled for boards where the NXP LPTMR
is enabled in the board devicetree.

Using this low-power timer as system timer only makes sense when using
power management. Otherwise, it just results in a lower tick resolution and
non-tickless operation.

Add dependency on CONFIG_PM for CONFIG_MCUX_LPTMR_TIMER.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>
(cherry picked from commit 63850804)