net: phy: mscc: split the driver into separate files

This patch splits the MSCC driver into separate files, per
functionality, to improve readability and maintenance as the codebase
grew a lot. The MACsec code is moved to a dedicated mscc_macsec.c file,
the mscc.c file is renamed to mscc_main.c to keep the driver binary to
be named mscc and common definition are put into a new mscc.h header.

Most of the code was just moved around, except for a few exceptions:
- Header inclusions were reworked to only keep what's needed.
- Three helpers were created in the MACsec code, to avoid #ifdef's in
  the main C file: vsc8584_macsec_init, vsc8584_handle_macsec_interrupt
  and vsc8584_config_macsec_intr.

The patch should not introduce any functional modification.

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