PCI: keystone: Use quirk to set MRRS for PCI host bridge

Reuse the already existing quirk to set MRRS for PCI host bridge
instead of explicitly setting MRRS in ks_pcie_host_init().

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