[MIPS] i8253: make the pit_clockevent variable static

The pit_clockevent symbol is needlessly defined global. This patch makes
that variable static.

Spotted by sparse. Compile-tested using Malta defconfig.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>