PCI: dwc: Define maximum number of vectors

Add a callback to define the maximum number of vectors used by the RC.

Since this is a parameter associated to each SoC IP setting, makes sense
to be configurable and easily visible to future modifications.

Set DesignWare driver vectors number maximum to 256.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>