[MIPS] Alchemy: Fix BCSR accesses.

    
Fixes BCSR accesses in the board setup/reset code.  The registers are
actually 16-bit, and their addresses are different between DBAu1550 and
other DBAu1xx0 boards.
    
From Sergei Shtylylov <sshtylyov@ru.mvista.com>.
    
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>