net: phy: mscc: configure both RX and TX internal delays for RGMII

The driver appears to be secretly enabling the RX clock skew
irrespective of PHY interface type, which is generally considered a big
no-no.

Make them configurable instead, and add TX internal delays when
necessary too.

While at it, configure a more canonical clock skew of 2.0 nanoseconds
than the current default of 1.1 ns.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>