mpc83xx: pcie: Read the clock from registers

The MPC83xx DM timer driver disables arch.pciexp*_clk, and uses
clk_get_rate instead. But the legacy MPC83xx PCIe driver still uses
arch.pciexp*_clk for the clock.

Hence, read the PCIe clock from the registers in the legacy MPC83xx PCIe
driver.

Signed-off-by: Mario Six <mario.six@gdsys.cc>