net: mvpp2: Split the PPv2 driver to a dedicated directory

As the mvpp2 driver is growing, move this driver to a dedicated
directory and split it into several files.

Since this driver has a lot of register defines and structure
definitions, it can benefit from having all of this into a dedicated
header file, named mvpp2.h.

A good chunk of the mvpp2 code is dedicated to Header Parser handling, so
we introduce mvpp2_prs.h where all Header Parser definitions are located,
and mvpp2_prs.c containing the related code.

In the same way, mvpp2_cls.h and mvpp2_cls.c are created to contain
Classifier and RSS related code.

The former 'mvpp2.c' file is renamed 'mvpp2_main.c' so that we can keep
the driver binary named 'mvpp2'.

This commit is only about spliting the driver into multiple files and
doesn't introduce any new function, feature or fix besides removing
'static' keywords when needed.

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