generic: permit support of standalone PCS for external kernel module

generic: permit support of standalone PCS for external kernel module

The current code permits support of the standalone PCS feature only for
in-tree kernel module but doesn't correctly support PCS from external
kernel module.

This is caused by the fact that the FWNODE_PCS config flag is internally
selected by any PCS driver and can't be selected directly. This is
problematic for any external kernel module that wants to use the standalone
PCS feature and needs the OPs provided by the generic PCS code.

Moreover compiling the standalone PCS code as a module is problematic and
would cause link error caused by the late PCS code that introduce a
notifier where phylink code depends on.

To address both problem, permit to select the FWNODE_PCS and change it to a
simple bool preventing it to compile as a module.

Link: https://github.com/openwrt/openwrt/pull/23349
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>