net: mvpp2: set the XPCS and MPCS in reset when not used

This patch sets both the XPCS and MPCS blocks in reset when they aren't
used. This is done both at boot time and when reconfiguring a port mode.
The advantage now is that only the PCS used is set out of reset when the
port is configured (10GKR uses the MCPS while RXAUI uses the XPCS).

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>