Blackfin arch: Fix bug - Kernel does not boot if re-program clocks

Don't write conflicting data to EBIU_SDBCTL after the SDRAM is
configured. This can cause data corruption, since we might change SDRAM
row and column addressing modes.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>