soc/intel_adsp: Refactor boot loader initialization path

Remove an unused strcmp() implementation.  Flatten the call tree for
HP-SRAM initialization for clarity.  Better isolate the platform
dependencies so e.g. hp_sram_pm_banks() becomes a clean noop on 1.5.

Also removes some dead/vestigial "error" handling, which wasn't being
propagated anywhere.  Note that error detection and handling is a bad
idea, but this is VERY early code.  We don't have even a theoretical
way of getting information back to the host until after SRAM is
initialized and window zero is set up.  (And even then there's no
protocol available other than signaling "FW_ENTERED" or... not).

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