tests/drivers/ipm: Enable on x86_64

this case isn't designed for SMP, setting CONFIG_MP_NUM_CPUS=1 to
run the full SMP kernel config, but with only one CPU available.

Fixes: #12478.

Signed-off-by: Wentong Wu <[email protected]>

Author: wentongwu

tests/drivers/ipm/prj.conf

@@ -5,3 +5,4 @@ CONFIG_STDOUT_CONSOLE=n
 CONFIG_IPM_CONSOLE_RECEIVER=y
 CONFIG_IPM_CONSOLE_SENDER=y
 CONFIG_IRQ_OFFLOAD=y
+CONFIG_MP_NUM_CPUS=1

tests/drivers/ipm/testcase.yaml

@@ -1,5 +1,4 @@
 tests:
   peripheral.mailbox:
     arch_exclude: posix xtensa
-    platform_exclude: qemu_x86_64 # see issue #12478
     tags: drivers ipc