ppc4xx: Only fixup opb attached UARTs

This patch updates the fdt UART clock fixup code to
only touch CPU internal UARTs on 4xx systems.
Only these UARTs are definitely clocked by gd->uart_clk.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Signed-off-by: Stefan Roese <sr@denx.de>