soc/intel_adsp: Add MP startup delay on cAVS 1.5

Investigation by Kai Vehmanen has shown that there is a very short
delay needed before starting the secondary core on cAVS 1.5 hardware.
What we finally realized is happening is that on these devices,
secondary core power is managed by the host.  The cavs-fw.py test
integration powers the second core on at system startup and lets
Zephyr start it later, but SOF will power it up and send an IPC to the
firmware immediately.

There is a period after power-up but before the ROM is available
(unclear whether this is a race vs. hardware, the ROM firmware, or the
kernel driver, or potentially some combination); interrupts latched
earlier than that seem to be cleared by CPU initialization.

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