net: mvpp2: set the GMAC, XLG MAC, XPCS and MPCS in reset when a port is down

This patch adds calls in the stop() helper to ensure both MACs and
both PCS blocks are set in reset when the user manually sets a port
down. This is done so that we have the exact same block reset states at
boot time and when a port is set down.

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