PCI: keystone: Use PCI_NUM_INTX

Switch from using custom MAX_LEGACY_IRQS and MAX_LEGACY_HOST_IRQS macros to
the generic PCI_NUM_INTX definition for the number of INTx interrupts.

Based-on-similar-patches-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>