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/README b/README
index 5ca11f2..89606c8 100644
--- a/README
+++ b/README
@@ -3153,6 +3153,7 @@
 sspi	- SPI utility commands
 base	- print or set address offset
 printenv- print environment variables
+pwm	- control pwm channels
 setenv	- set environment variables
 saveenv - save environment variables to persistent storage
 protect - enable or disable FLASH write protection