hwinfo: gecko: fix warning when building for efr32bg27_brd2602a

When building the driver for efr32bg27_brd2602a none of the condition
matches and the compiler warns for:

hwinfo_gecko.c: In function 'z_impl_hwinfo_get_reset_cause':
hwinfo_gecko.c:38:18: warning: unused variable 'rmu_flags'
   38 |         uint32_t rmu_flags = RMU_ResetCauseGet();

This fails the bi-weekly build, add a __maybe_unused to silence the
warning.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>