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>
---
M src/mainboard/google/eve/devicetree.cb
M src/mainboard/google/fizz/variants/baseboard/devicetree.cb
M src/mainboard/google/glados/variants/asuka/devicetree.cb
M src/mainboard/google/glados/variants/caroline/devicetree.cb
M src/mainboard/google/glados/variants/cave/devicetree.cb
M src/mainboard/google/glados/variants/chell/devicetree.cb
M src/mainboard/google/glados/variants/glados/devicetree.cb
M src/mainboard/google/glados/variants/lars/devicetree.cb
M src/mainboard/google/glados/variants/sentry/devicetree.cb
M src/mainboard/google/poppy/variants/atlas/devicetree.cb
M src/mainboard/google/poppy/variants/baseboard/devicetree.cb
M src/mainboard/google/poppy/variants/nami/devicetree.cb
M src/mainboard/google/poppy/variants/nautilus/devicetree.cb
M src/mainboard/google/poppy/variants/nocturne/devicetree.cb
M src/mainboard/google/poppy/variants/rammus/devicetree.cb
M src/mainboard/google/poppy/variants/soraka/devicetree.cb
M src/mainboard/intel/kblrvp/variants/baseboard/devicetree.cb
M src/mainboard/intel/kblrvp/variants/rvp11/overridetree.cb
M src/mainboard/intel/kblrvp/variants/rvp8/overridetree.cb
M src/mainboard/intel/kunimitsu/devicetree.cb
M src/mainboard/intel/saddlebrook/Kconfig
M src/mainboard/intel/saddlebrook/devicetree.cb
M src/mainboard/purism/librem_skl/Kconfig
M src/mainboard/purism/librem_skl/variants/librem13v2/devicetree.cb
M src/mainboard/purism/librem_skl/variants/librem15v3/devicetree.cb
M src/soc/intel/skylake/Kconfig
M src/soc/intel/skylake/chip.c
M src/soc/intel/skylake/chip.h
M src/soc/intel/skylake/chip_fsp20.c
M src/soc/intel/skylake/lpc.c
30 files changed, 22 insertions(+), 52 deletions(-)