sb/intel/i82801gx: Add common early code

sb/intel/i82801gx: Add common early code

Remove some of the code duplication on i82801gx.
x4x boards are left untouched for now since that northbridge
also supports i82801jx.

The order of some things has changed:
- on i945 early_ich7_init is now done before the raminit
- enabling the IOAPIC is done before the raminit

Change-Id: Ie39549938891e17667a8819b49a78b9c71c8ec9e
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36754
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>