i2c: mediatek: add bus regulator control for power saving

Add conditional bus regulator enable/disable in mtk_i2c_transfer()
to support I2C bus power gating for platforms that require it.

This implementation:
- Enables bus_regulator before clk_bulk_enable() if vbus-supply is defined
- Disables bus_regulator after clk_bulk_disable()
- Only activates when vbus-supply is provided in device tree
- Has no impact on platforms without vbus-supply defined

This approach provides power savings for platforms with an extra I2C bus
regulator, while avoiding runtime PM complexity.

Tested on MT8188.

Signed-off-by: Adlavinitha Reddy <adlavinitha.reddy@mediatek.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260318084621.4127757-2-adlavinitha.reddy@mediatek.com

drivers/i2c/busses/i2c-mt65xx.c

@@ -21,6 +21,7 @@
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
+#include <linux/regulator/consumer.h>
 #include <linux/scatterlist.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
@@ -1244,9 +1245,15 @@ static int mtk_i2c_transfer(struct i2c_adapter *adap,
 	bool write_then_read_en = false;
 	struct mtk_i2c *i2c = i2c_get_adapdata(adap);
 
+	if (i2c->adap.bus_regulator) {
+		ret = regulator_enable(i2c->adap.bus_regulator);
+		if (ret)
+			return ret;
+	}
+
 	ret = clk_bulk_enable(I2C_MT65XX_CLK_MAX, i2c->clocks);
 	if (ret)
-		return ret;
+		goto err_regulator;
 
 	i2c->auto_restart = i2c->dev_comp->auto_restart;
 
@@ -1301,6 +1308,10 @@ static int mtk_i2c_transfer(struct i2c_adapter *adap,
 
 err_exit:
 	clk_bulk_disable(I2C_MT65XX_CLK_MAX, i2c->clocks);
+err_regulator:
+	if (i2c->adap.bus_regulator)
+		regulator_disable(i2c->adap.bus_regulator);
+
 	return ret;
 }