soc/intel/xeon_sp: Move set_bios_init_completion()

soc/intel/xeon_sp: Move set_bios_init_completion()

Move set_bios_init_completion() and helper functions from skx
and cpx soc_util.c to xeon common util.c. There are some slight
differences between skx and cpx, so used the more correct cpx
functions. Both cpx and skx platforms boot as expected.

Change-Id: Ie416b3a43ccdd14a0eb542786593c2eb4d37450f
Signed-off-by: Marc Jones <marcjones@sysproconsulting.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47172
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Jay Talbott <JayTalbott@sysproconsulting.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>