board: intel_adsp_ace15_mtpm: Disable MTL IMR context save by default

Disable IMR context save on MTL by default, as it isn't a mandatory
feature for MTL.

The feature implementation is kept. So users can try this feature by
setting CONFIG_ADSP_IMR_CONTEXT_SAVE=y

Signed-off-by: Mengdong Lin <mengdong.lin@intel.com>

Author: mengdonglin

app/boards/intel_adsp_ace15_mtpm.conf

@@ -18,7 +18,7 @@ CONFIG_PM_POLICY_CUSTOM=y
 CONFIG_POWER_DOMAIN=y
 CONFIG_POWER_DOMAIN_INTEL_ADSP=y
 
-CONFIG_ADSP_IMR_CONTEXT_SAVE=y
+CONFIG_ADSP_IMR_CONTEXT_SAVE=n
 
 # enable Zephyr drivers
 CONFIG_ZEPHYR_NATIVE_DRIVERS=y