phy: cdns-dphy: Prepare for Rx support

The Rx programming sequence differs from the Tx programming sequence.
Currently only Tx mode is supported. Move all the Tx related parts into
a set of Tx-specific hooks that are then called by the main PHY
framework hooks. This way when Rx support is added all that is needed to
be done is to plug in the Rx hooks.

The clocks "psm" and "pll_ref" are not used by the Rx path so make them
optional in the probe and then check if they exist in the power_on()
hook.

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