drivers: eth: gmac: Proper ptp clock setup

Calculate proper initial PTP clock divisors based on the MCK value.

Additionally do not allow adjusting the rate of the clock. This does not
seem to work properly with current gPTP rate adjustment algorithm.

Having proper PTP clock divisors and callbacks that allow getting,
setting and adjusting current time is sufficient for proper gPTP
support.

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>