mpc83xx: Add PCI-E support for MPC837XEMDS boards

MPC837XEMDS boards can support PCI-E via "PCI-E riser card". The card
provides two PCI-E (x2) ports. Though, only one port can be used in x2
mode. Two ports can function simultaneously in x1 mode.

PCI-E x1/x2 modes can be switched via "pex_x2" environment variable.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>