net: stmmac: dwmac-meson8b: Make the clock enabling code re-usable

The timing adjustment clock will need similar logic as the RGMII clock:
It has to be enabled in the driver conditionally and when the driver is
unloaded it should be disabled again. Extract the existing code for the
RGMII clock into a new function so it can be re-used.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>