MIPS: JZ4740: Add timer support

Add support for the timer/counter unit on a JZ4740 SoC.  This code is used
as a common base for the JZ4740 clocksource/clockevent implementation and
PWM support.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1396/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>