boards: silabs: siwx91x: Fix PM with rb4342

Currently twister complains:

    benchmark.kernel.footprints.pm on siwx917_rb4342a/siwg917m111mgtba
    error (CMake build failure - warning: SILABS_SLEEPTIMER_TIMER (defined
    at soc/silabs/silabs_s2/Kconfig.defconfig:17,
    drivers/timer/Kconfig.silabs:4) has direct dependencies
    (SOC_FAMILY_SILABS_S2 || SOC_FAMILY_SILABS_SIWX91X) &&
    DT_HAS_SILABS_GECKO_STIMER_ENABLED && SYS_CLOCK_EXISTS with value n,
    but is currently being y-selected by the following symbols:)

So, SILABS_SLEEPTIMER_TIMER can't be selected because
DT_HAS_SILABS_GECKO_STIMER_ENABLED is not selected.

We just have to enable the proper timer in the DT of rb4342s to fix the
issue.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>