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