Conversion of the driver to gpiod API done in 468ba54b ("fec:
convert to gpio descriptor") incorrectly made reset line mandatory and
resulted in aborting driver probe in cases where reset line was not
specified (note: this way of specifying PHY reset line is actually
deprecated).

Switch to using devm_gpiod_get_optional() and skip manipulating reset
line if it can not be located.

Fixes: 468ba54b ("fec: convert to gpio descriptor")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Tested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230201215320.528319-1-dmitry.torokhov@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>