MIPS: ingenic: CI20: enable OST, PWM drivers in defconfig

The OST driver provides a clocksource and sched_clock that are much more
accurate than the default ones.

The PWM driver allows to use the PWM pins on the external header of the
board.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>