PCI: keystone: Cleanup error_irq configuration

pci-keystone driver uses irq_of_parse_and_map() to get irq number of
error_irq.

Use platform_get_irq() instead and move platform_get_irq() and
request_irq() of error_irq from ks_pcie_add_pcie_port to ks_pcie_probe
since error_irq is common to both RC mode and EP mode.

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