MIPS: spinlock: Adjust arch_spin_lock back-off time

Make it similar to the trylock and R10000_LLSC_WAR cases.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9789/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>