intel_adsp: ace: power: fix build error with asserts enabled

If defined(CONFIG_ASSERT) and !defined(CONFIG_ADSP_IMR_CONTEXT_SAVE),
build will fail as symbol "global_imr_ram_storage" is not defined.

Link: https://github.com/thesofproject/sof/issues/6896


Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>