baytrail: Factor out INT15 handler

The handler is the same on all Bay Trail mainboards. Factor it out.

Change-Id: Ia1b6faaca4792cda5f14948d23498182bf4bb2c3
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54415
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Benjamin Doron <benjamin.doron00@gmail.com>
Reviewed-by: Máté Kukri <kukri.mate@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/baytrail/Makefile.inc

@@ -48,6 +48,7 @@ ramstage-y += smm.c
 ramstage-y += southcluster.c
 ramstage-y += xhci.c
 ramstage-$(CONFIG_ELOG) += elog.c
+ramstage-$(CONFIG_VGA_ROM_RUN) += int15.c
 
 ifeq ($(CONFIG_HAVE_REFCODE_BLOB),y)
 ramstage-y += refcode.c