[MIPS] BCM47xx: Fix build error due to missing PCI functions

This patch defines pcibios_map_irq() and pcibios_plat_dev_init() for
the BCM47xx platform.

It fixes the regression introduced by commit
aab547ce.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>