dt-bindings: phy: mediatek,tphy: add property to set pre-emphasis

Add a property to set usb2 phy's pre-emphasis, which used to widen eye
opening and boost eye swing.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20220914060746.10004-2-chunfeng.yun@mediatek.com


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