cadence_gem: Add support for screening

cadence_gem: Add support for screening

The Cadence GEM hardware allows incoming data to be 'screened' based on some
register values. Add support for these screens.

We also need to increase the max regs to avoid compilation failures. These new
registers are implemented in the next patch.

Signed-off-by: Alistair Francis <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-id: 73e69a8ad9fa2763e9f68f71eaf2469dd5744fcc.1469727764[email protected]
Signed-off-by: Peter Maydell <[email protected]>