MIPS: sibyte: Declare the cfe_write() buffer as constant

MIPS: sibyte: Declare the cfe_write() buffer as constant

The write() prototype expects a const char * as argument so declare
it as such.

Fixes the following build problem:

arch/mips/sibyte/common/cfe_console.c:23:5: error: passing argument 2 of
'cfe_write' discards 'const' qualifier from pointer target type [-Werror]
arch/mips/sibyte/common/cfe_console.c:34:4: error: passing argument 2 of
'cfe_write' makes pointer from integer without a cast [-Werror]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: sibyte-users@bitmover.com
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5485/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>