phy: cadence: sierra: Model reference receiver as clocks (gate clocks)

Sierra has two reference recievers REFRCV and REFRCV1. REFRCV is used to
drive reference clock cmn_refclk_m/p to PLL_CMNLC1 and REFRCV1 is used to
drive reference clock cmn_refclk1_m/p to PLL_CMNLC. Model these
reference receivers as clocks in order for PLL_CMNLC and PLL_CMNLC1 to
be able to seamlessly use any of the external reference clocks.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>