PCI: dwc: Expand maximum number of MSI IRQs from 32 to 256

The Synopsys PCIe Root Complex supports up to MSI 256 IRQs distributed
over 8 controller registers, therefore the maximum number of MSI IRQs
can be changed to 256. The number of controllers can be calculated based
on the number of vectors used by the specific SoC driver.

Update the dwc host bridge driver maximum number of supported MSI
IRQs.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Tested-by: Niklas Cassel <niklas.cassel@axis.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>