mpc83xx: Replace CONFIG_83XX_CLKIN in calculations

CONFIG_SYS_CLK_FREQ is the standard way to set the system clock
frequency. On MPC83xx, CONFIG_83XX_CLKIN is used for this purpose.
Hence, the obvious way is to replace CONFIG_83XX_CLKIN with
CONFIG_SYS_CLK_FREQ.

A few MPC83xx boards use the CONFIG_83XX_CLKIN variable for computing
CONFIG_SYS_NS16550_CLK. This makes it harder to replace
CONFIG_83XX_CLKIN.

But the value of the multiplicator can be read from the SPMR register.

Hence, replace the static calculations with a call to a new get_bus_freq
function, as other architectures do.

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