ar71xx: move PB42 specific PCI init code into a separate file

target/linux/ar71xx/config-2.6.30

target/linux/ar71xx/config-2.6.31

target/linux/ar71xx/config-2.6.32

target/linux/ar71xx/files/arch/mips/ar71xx/Kconfig

target/linux/ar71xx/files/arch/mips/ar71xx/Makefile

target/linux/ar71xx/files/arch/mips/ar71xx/dev-pb42-pci.c

target/linux/ar71xx/files/arch/mips/ar71xx/dev-pb42-pci.h

target/linux/ar71xx/files/arch/mips/ar71xx/mach-aw-nr580.c

target/linux/ar71xx/files/arch/mips/ar71xx/mach-pb42.c

target/linux/ar71xx/files/arch/mips/ar71xx/mach-pb44.c

target/linux/ar71xx/files/arch/mips/ar71xx/mach-tl-wr841n.c

target/linux/ar71xx/files/arch/mips/ar71xx/mach-ubnt.c

target/linux/ar71xx/files/arch/mips/ar71xx/mach-wp543.c