ace: fix DSP panic during startup

pm_device_runtime_get() must be called after pd_intel_adsp_init() is
called for each device, because the latter calls
pm_device_runtime_enable(), which sets the device runtime PM use
count to 0. The current wrong calling order causes a DSP panic
because of an unbalanced pm_device_runtime_put(). Fix this by
delaying pm_device_runtime_get() until the POST_KERNEL initialisation
step.

Fixes commit c3a6274b ("intel_adsp: ace: power: Prevent HST
domain power gating")

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>