phy: ti: phy-j721e-wiz: set PMA_CMN_REFCLK_DIG_DIV based on reflk rate

For J7200-SR2.0 and AM64 we don't model Common refclock divider as
a clock divider as the divisor rate is fixed based on operating
reference clock frequency. We just program the recommended value
into the register. This simplifies the device tree and implementation
a lot.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>