drm/i915/bios: Extract soc/intel_rom.c

Abstract away the nuts and bolts of the SPI vs. PCI ROM
stuff, and hide it all in soc/intel_rom.c so that the
VBT code doesn't have to care about this stuff.

This leaves intel_bios.c with a single codepath that
can focus on the details related to the VBT layout.

This should have no functional changes.

v2: Rebase due to vbt_signature changes
    Drop unnecessary cast (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240923152453.11230-7-ville.syrjala@linux.intel.com