src/soc/ibm/power/Makefile.inc: Adding istep_8_7.c to romstage

Signed-off-by: Mateusz Kowalski <mateusz.kowalski@3mdeb.com>

Author: mmkow

src/soc/ibm/power9/Makefile.inc

@@ -6,6 +6,7 @@ bootblock-y += bootblock.c
 postcar-y += exit_car.S
 romstage-y += romstage.c
 romstage-y += istep_8_6.c
+romstage-y += istep_8_7.c
 ramstage-y += chip.c
 ramstage-y += timer.c