[POWERPC] Make pci_read_irq_line the default on mpc7448hpc2 board

The following patch adds a tsi108/9 pci interrupt controller host.
On mpc7448hpc2 board, pci_irq_fixup function is removed, which makes the
pci_read_irq_line be the default pci irq fixup.

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>