mgb: Connect if_mgb module to the build

It supports the following Microchip devices:

LAN7430 PCIe Gigabit Ethernet controller with PHY
LAN7431 PCIe Gigabit Ethernet controller with RGMII interface

The driver has a number of caveats and limitations, but is functional.

Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation

share/man/man4/Makefile

@@ -297,6 +297,7 @@ MAN=	aac.4 \
 	mem.4 \
 	meteor.4 \
 	mfi.4 \
+	${_mgb.4} \
 	miibus.4 \
 	mld.4 \
 	mlx.4 \
@@ -816,6 +817,7 @@ _igc.4=		igc.4
 _imcsmb.4=	imcsmb.4
 _io.4=		io.4
 _itwd.4=	itwd.4
+_mgb.4=		mgb.4
 _nda.4=		nda.4
 _nfe.4=		nfe.4
 _nfsmb.4=	nfsmb.4

sys/modules/Makefile

@@ -232,6 +232,7 @@ SUBDIR=	\
 	${_mana} \
 	md \
 	mdio \
+	${_mgb} \
 	mem \
 	mfi \
 	mii \
@@ -652,6 +653,7 @@ _ixv=		ixv
 _lio=		lio
 .endif
 _mana=		mana
+_mgb=		mgb
 _nctgpio=	nctgpio
 _ntb=		ntb
 _ocs_fc=	ocs_fc