pm: use braces for .pm_base inside anonymous union initializer

Fixes commit 25173f71 ("pm: device_runtime: Extend with synchronous
runtime PM")

Fixes compilation with gcc 4.2-based toolchain used by SOF for TGL
generation products. As seen in the error message below, that gcc
version requires braces for initialization of anonymous unions:

```
zephyr/soc/xtensa/intel_adsp/common/mem_window.c:62:
   error: unknown field ‘pm_base’ specified in initializer
   warning: missing braces around initializer
   warning: (near initialization for ‘__device_dts_ord_66.<anonymous>’)
```

This is a well-known and recurring issue, see past example(s) in #68118

As the Zephyr build is deterministic, I could easily verify that this
commit makes zero .obj difference (when using the Zephyr SDK).

Signed-off-by: Marc Herbert <marc.herbert@intel.com>