Missing DKMS support

[TheFunctionalGuy]

Motivation / Context

Currently using this as a kernel module is a little bit of a hassle as it has to be compiled manually again when a newer kernel is installed.

Describe the solution you'd like

Here it would come in handy when DKMS support is added.
This then would enable automatic re-compilation and loading after an kernel update.

I already have a working version of this lying around on my machine.

In case this is wanted I can update the README.md with instructions on how to use this with the dkms command.

dkms.conf:

PACKAGE_NAME="gtp5g"
PACKAGE_VERSION="0.8.10"

CLEAN="make clean KVER=$kernelver"
MAKE="make KVER=$kernelver"

BUILT_MODULE_NAME="gtp5g"
DEST_MODULE_LOCATION="/kernel/drivers/net"

AUTOINSTALL="yes"

If you want to I can open a pull request for this changes.

Cheers.