drivers: timer: lptim: depend on SYS_POWER_MANAGEMENT

The LPTIM driver is supposed to be only available when the SoC is
allowed to enter power sleep mode, as described in commit f30f5fff
("drivers: timer: lptim is [EXPERIMENTAL] for stm32 soc series only").

For that it should depends on SYS_POWER_MANAGEMENT (which gates the
SYS_POWER_SLEEP_STATES and SYS_POWER_DEEP_SLEEP_STATES options) instead
of DEVICE_POWER_MANAGEMENT.

Fixes #25989

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>