net: phy: allow a phy to opt-out of interrupt handling

Until now, it is not possible for a PHY driver to disable interrupts
during runtime. If a driver offers the .config_intr() as well as the
.handle_interrupt() ops, it is eligible for interrupt handling.
Introduce a new flag for the dev_flags property of struct phy_device, which
can be set by PHY driver to skip interrupt setup and fall back to polling
mode.

At the moment, this is used for the MaxLinear PHY which has broken
interrupt handling and there is a need to disable interrupts in some
cases.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>