pinctrl: mediatek: fix failing to get syscon

pinctrl: mediatek: fix failing to get syscon

Replace uclass_get_device_by_ofnode() with syscon_regmap_lookup_by_phandle()
to get the "mediatek,pctl-regmap" syscon device.

Depending on probe order, uclass_get_device_by_ofnode() may fail, but
syscon_regmap_lookup_by_phandle() has logic in it to handle that case
correctly.

The previous implementation could read more than one syscon if the
"mediatek,pctl-regmap" property had more than one phandle, but the one
board with a devicetree that does that is not supported in U-Boot yet,
so we can save that for later (it may never be needed).

Fixes: 424ceba18bfb ("pinctrl: mediatek: support mediatek,pctl-regmap property")
Signed-off-by: David Lechner <dlechner@baylibre.com>