realtek: add PWM support

Enable CONFIG_PWM and CONFIG_PWM_SYSFS on the RTL839x target. This is
required to support PWM-controlled devices described in the Device Tree,
such as generic PWM fans (kmod-hwmon-pwmfan).

Enabling the PWM core framework introduces a minimal footprint overhead.
Impact measured on the edgecore_ecs4100-12ph profile:

- Flash (compressed kernel .bin) : +5.4 KB
- RAM (vmlinux bloat-o-meter)    : +18.3 KB

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>

Author: luizluca

target/linux/realtek/rtl839x/config-6.18

@@ -217,6 +217,8 @@ CONFIG_POWER_RESET=y
 CONFIG_POWER_RESET_GPIO_RESTART=y
 CONFIG_POWER_RESET_SYSCON=y
 CONFIG_PTP_1588_CLOCK_OPTIONAL=y
+CONFIG_PWM=y
+CONFIG_PWM_SYSFS=y
 CONFIG_QUEUED_RWLOCKS=y
 CONFIG_QUEUED_SPINLOCKS=y
 CONFIG_RANDSTRUCT_NONE=y