net: mscc: describe the PTP register range

This patch adds support for using the PTP register range, and adds a
description of its registers. This bank is used when configuring PTP.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>