sb/intel/lynxpoint: Enable/disable AER via Kconfig

sb/intel/lynxpoint: Enable/disable AER via Kconfig

Several changes[1][2] to the Linux kernel now enable ASPM/AER for the
rt8169 network driver, for which it was previously disabled. This,
coupled with the southbridge enabling AER for all PCIe devices, has
resulted in a large amount of AER timeout errors in the kernel log for
boards which utilize the rt8169 for on-board Ethernet (e.g., google/beltino).
While performance is not impacted, the errors do accumulate.

To mitigate this, guard AER enablement via Kconfig, select it by default
(as to maintain current default behavior), and allow boards which need
to disable it to do so (implemented in subsequent commits).

This implementation is derived from that in soc/intel/broadwell.

Test: build/boot google/beltino variants with AER disabled (CB:46136),
verify dmesg log free of AER timeout errors.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=671646c151d492c3846e6e6797e72ff757b5d65e
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a99790bf5c7f3d68d8b01e015d3212a98ee7bd57

Change-Id: Ia03ef0d111335892c65122954c1248191ded7cb8
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46133
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>