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>
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