Renesas Ethernet AVB PTP clock driver

Ethernet AVB device includes the gPTP  timer, so we can implement a PTP clock
driver.  We're doing that in a separate file, with  the main Ethernet driver
calling the PTP driver's [de]initialization and interrupt handler functions.
Unfortunately, the clock seems tightly coupled with the AVB-DMAC, so when that
one leaves the operation mode, we have to unregister the PTP clock... :-(

Based on the original patches by Masaru Nagai.

Signed-off-by: Masaru Nagai <masaru.nagai.vx@renesas.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>