phy: phy-cadence-salvo: add phy .init API

The .init is used for one-time PHY's initialization, and .power_on
is called many times during the device lifecycle.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/20200513125605.5545-1-peter.chen@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>