arm64: Add netmap(4) to default kernel config

Allows the development, testing and deployment of netmap(4)-based code
on arm64 without having to recompile the kernel. netmap(4) is already
in the amd64 and powerpc64 default configs, so it does not seem
unreasonable to also provide it on arm64 by default.

Note that netmap(4) is useful even on systems without NIC that fully
support it.

Reviewed by: vmaffione
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D43702