soc/intel/skylake: Unify serial IRQ options

soc/intel/skylake: Unify serial IRQ options

We had two ways to configure the serial IRQ mode. One time in the
devicetree for FSP and one time through Kconfig for coreboot. We'll
use `enum serirq_mode` from soc/intel/common/ as a devicetree option
instead. As the default is `quiet mode` here and that is the most
common mode, this saves us a lot of lines.

In four cases kblrvp8, 11 and librem 13v2, 15v3, we had conflicting
settings in devicetree and Kconfig. We'll maintain the `continuous`
selection, although it might be that coreboot overrode this earlier
on the kblrvps.

Note: A lot of Google boards have serial IRQ enabled, while the pin
seems to be unconnected?

Change-Id: I79f0cd302e335d8dcf8bf6bc32f3d40ca6713e5c
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/31596
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>