net: phy: mediatek: do not require syscon compatible for pio property

Current implementation requires syscon compatible for pio property
which is used for driving the switch leds on mt7988.

Replace syscon_regmap_lookup_by_phandle with of_parse_phandle and
device_node_to_regmap to get the regmap already assigned by pinctrl
driver.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Link: https://patch.msgid.link/20250510174933.154589-1-linux@fw-web.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>