MAINTAINERS: Include Samsung PWM in Samsung SoC entry

The Samsung PWM and PWM-based timer/clocksource drivers lacked dedicated
maintainers entry.  They are used on all Samsung SoC designs (although
timer/clocksource driver only on older platforms), so include them in
Samsung SoC entry maintained by Krzysztof Kozlowski.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210506202729.157260-5-krzysztof.kozlowski@canonical.com