net: stmmac: add stmmac_setup_ptp()

Add a function to setup PTP, which will enable the clock, initialise
the timestamping, and register with the PTP clock subsystem. Call this
when we want to register the PTP clock in stmmac_hw_setup(), otherwise
just call the

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>