soc/intel/broadwell/systemagent.c: Rename to `northbridge.c`

Change-Id: Id1a0e02174456bb25df0721cfd3865645641a01a
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46797
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/soc/intel/broadwell/Makefile.inc

@@ -17,11 +17,11 @@ ramstage-y += memmap.c
 romstage-y += memmap.c
 postcar-y += memmap.c
 ramstage-y += minihd.c
+ramstage-y += northbridge.c
 ramstage-y += pei_data.c
 romstage-y += pei_data.c
 ramstage-y += ramstage.c
 ramstage-$(CONFIG_HAVE_REFCODE_BLOB) += refcode.c
-ramstage-y += systemagent.c
 
 CPPFLAGS_common += -Isrc/soc/intel/broadwell/include