phy: phy-mtk-tphy: add property to set pre-emphasis

Add a property to set usb2 phy's pre-emphasis, it's disabled by default
on some SoCs.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220914060746.10004-3-chunfeng.yun@mediatek.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>