net: ethernet: renesas: rcar_gen4_ptp: Break out to module

The Gen4 gPTP support will be shared between the existing Renesas
Ethernet Switch driver and the upcoming Renesas Ethernet-TSN driver. In
preparation for this break out the gPTP support to its own module.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>