boards/intel_adsp: Update cavs-fw.py, robustify vs. Linux PM

Add support for the extended manifest blocks that recent versions of
rimage are including as a prefix on the firmware blob.

Also include some PCI runtime PM tricks to reliably enable the DSP
device on kernels that have turned off the DSP device automatically
(e.g. systems that have PM enabled by default but where the SOF driver
is not loaded -- chromebooks work this way, potentially other
distros).

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>