PCI: keystone: Add separate functions for configuring MSI and legacy interrupt

ks_pcie_get_irq_controller_info() is used to configure both MSI and
legacy interrupt. This will prevent MSI or legacy interrupt specific
intializations. Add separate functions to configure MSI and legacy
interrupts.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>