Blackfin: dynamically update UART speed when initializing

Previously, booting over the UART required the baud rate to be known ahead
of time.  Using a bit of tricky simple math, we can calculate the new board
rate based on the old divisors.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>