net: pcs: xpcs: move register bit descriptions to a header file

Vendors which integrate the Designware XPCS might modify a few things
here and there, and to support those, it's best to create separate C
files in order to not clutter up the main pcs-xpcs.c.

Because the vendor files might want to access the common xpcs registers
too, let's move them in a header file which is local to this driver and
can be included by vendor files as appropriate.

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