[MIPS] MT: Initialise all writable bits in Cause register to zero.

Recent 34Ks come out of reset with WP enabled on VPE 1 so we take an
immediate exception when starting the second VPE.

Signed-off-by: Chris Dearman <chris@mips.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>