8xxx-fdt: set ns16550 clock from CFG_NS16550_CLK, not bi_busfreq

Some boards that have external 16550 UARTs don't have a direct
tie between bi_busfreq and the clock used for the UARTs.  Boards
that do have such a tie should set CFG_NS16550_CLK to be
get_bus_freq(0) -- which most of them do already.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>