Minor cleanup to clock-related defines for DaVinci DM6446 boards:

 - CONFIG_SYS_CLK_FREQ is unused; remove it.

 - CONFIG_SYS_NS16550_CLK must be the same as CONFIG_SYS_HZ_CLOCK

On DM6446 both of those peripheral clocks actually come from the
same source, the primary oscillator.  Having them use the same
symbol avoids bugs in the clone'n'modify development cycle.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>