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

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