MPC83XX: Fix PCI express clock setup

On a 8308 based board it was found that the PEX_GLK_RATIO register
(programmed in arch/powerpc/cpu/mpc83xx/pcie.c) was getting set to 0, This
was tracked to the fact that the pci express clock frequency was not being
assigned to the pciexp1_clk entry in the global data structure in file
arch/powerpc/cpu/mpc83xx/speed.c. Fix this and a similiar issue in
'do_clocks' command.

Signed-off-by: Bill Cook <cook@isgchips.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>