[MIPS] time: Add GT641xx timer0 clockevent driver

And make use of it for Cobalt.  A few others such as the Malta could make
use of it as well.

Signed-off-by: Yoichi Yuasa <yoichi_yuasa@tripeaks.co.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>