pwm: Add PWM driver for SiFive SoC

Adds a PWM driver for PWM chip present in SiFive's HiFive Unleashed SoC
This driver is simple port of Linux pwm sifive driver from Linux v5.6

commit: 9e37a53eb051 ("pwm: sifive: Add a driver for SiFive SoC PWM")

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
index 2c3a069..0f4e84b 100644
--- a/drivers/pwm/Makefile
+++ b/drivers/pwm/Makefile
@@ -15,5 +15,6 @@
 obj-$(CONFIG_PWM_MTK)		+= pwm-mtk.o
 obj-$(CONFIG_PWM_ROCKCHIP)	+= rk_pwm.o
 obj-$(CONFIG_PWM_SANDBOX)	+= sandbox_pwm.o
+obj-$(CONFIG_PWM_SIFIVE)	+= pwm-sifive.o
 obj-$(CONFIG_PWM_TEGRA)		+= tegra_pwm.o
 obj-$(CONFIG_PWM_SUNXI)		+= sunxi_pwm.o