phy: cdns-dphy: Allow setting mode

Allow callers to set the PHY mode. The main mode should always be
PHY_MODE_MIPI_DPHY but the submode can either be
PHY_MIPI_DPHY_SUBMODE_RX or PHY_MIPI_DPHY_SUBMODE_TX. Update the ops
based on the requested submode.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>