testbench: test multiple patterns.

Make sure that we aren't relying on original contents.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
bridge: add a simple 32-bit to 16-bit bridge.

Add a 32-bit bridge so that we can connect this to a 32-bit data bus and
not have the master worrying about splitting accesses.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
sdram: optimize the autorefresh period.

Rather than an arbitrary tRef - 16 cycles, work out how often we really
need to issue to autorefresh command.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>