net: stmmac: allow platforms to use PHY tx clock stop capability

Allow platform glue to instruct stmmac to make use of the PHY transmit
clock stop capability when deciding whether to allow the transmit clock
from the DWMAC core to be stopped.

Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tsITp-005vG9-Px@rmk-PC.armlinux.org.uk


Signed-off-by: Paolo Abeni <pabeni@redhat.com>