powerpc/83xx: Add PCI-E support for all MPC83xx boards with PCI-E

This patch adds pcie nodes to the appropriate dts files, plus adds
some probing code for the boards.

Also, remove of_device_is_avaliable() check from the mpc837x_mds.c
board file, as mpc83xx_add_bridge() has the same check now.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>