net: pcs: Add 10GBASE-R mode for Synopsys Designware XPCS

Add basic support for XPCS using 10GBASE-R interface. This mode will
be extended to use interrupt, so set pcs.poll false. And avoid soft
reset so that the device using this mode is in the default configuration.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>