cmd: Add a pwm command

Add the command "pwm" for controlling the pwm channels. This
command provides pwm invert/config/enable/disable functionalities
via PWM uclass drivers

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/cmd/Makefile b/test/cmd/Makefile
index b2f71af..758bc14 100644
--- a/test/cmd/Makefile
+++ b/test/cmd/Makefile
@@ -4,4 +4,5 @@
 
 obj-y += mem.o
 obj-$(CONFIG_CMD_MEM_SEARCH) += mem_search.o
+obj-$(CONFIG_CMD_PWM) += pwm.o
 obj-y += setexpr.o