cgem: Support building as a loadable kernel module

For development, building a driver as kernel module is both convenient
and a time saver (no need for reboot on some change, testing it requires
just kldunload and kldload, a matter of seconds). For some special
cases, it may be even desirable to postpone initializing the network
interface after some action is done (loading a FPGA bitstream may be
required for Zynq/ZynqMP based hardware as an example).

Building is limited to ARM, ARM64 and RISC-V architectures (for Zynq,
ZynqMP, PolarFire Soc based boards, and HiFive based boards are known to
use CGEM at the moment).

Reviewed by:	mhorne
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D34687

sys/modules/Makefile

@@ -147,6 +147,7 @@ SUBDIR=	\
 	${_ice_ddp} \
 	${_ida} \
 	if_bridge \
+	${_if_cgem} \
 	if_disc \
 	if_edsc \
 	${_if_enc} \
@@ -575,6 +576,11 @@ _ice_ddp=	ice_ddp
 .endif
 .endif
 
+.if ${MACHINE_CPUARCH} == "aarch64" || ${MACHINE_CPUARCH} == "arm" || \
+	${MACHINE_CPUARCH} == "riscv"
+_if_cgem=	if_cgem
+.endif
+
 # These rely on 64bit atomics
 .if ${MACHINE_ARCH} != "powerpc" && ${MACHINE_ARCH} != "powerpcspe"
 _mps=		mps

sys/modules/if_cgem/Makefile

@@ -0,0 +1,7 @@
+.PATH: ${SRCTOP}/sys/dev/cadence
+
+KMOD=	if_cgem
+SRCS=	if_cgem.c
+SRCS+=	bus_if.h device_if.h miibus_if.h ofw_bus_if.h
+
+.include <bsd.kmod.mk>