As a first step into making phylib and all PHY drivers to actually
have support for shared IRQs, make the .ack_interrupt() callback
optional.

After all drivers have been moved to implement the generic
interrupt handle, the phy_drv_supports_irq() check will be
changed again to only require the .handle_interrupts() callback.

Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Andre Edich <andre.edich@microchip.com>
Cc: Antoine Tenart <atenart@kernel.org>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: Divya Koppera <Divya.Koppera@microchip.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Kavya Sree Kotagiri <kavyasree.kotagiri@microchip.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Marco Felsch <m.felsch@pengutronix.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: Mathias Kresin <dev@kresin.me>
Cc: Maxim Kochetkov <fido_max@inbox.ru>
Cc: Michael Walle <michael@walle.cc>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Nisar Sayed <Nisar.Sayed@microchip.com>
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Philippe Schenker <philippe.schenker@toradex.com>
Cc: Willy Liu <willy.liu@realtek.com>
Cc: Yuiko Oshino <yuiko.oshino@microchip.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>