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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 58d4ef1..86dc603 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -58,6 +58,7 @@
 CONFIG_CMD_MUX=y
 CONFIG_CMD_OSD=y
 CONFIG_CMD_PCI=y
+CONFIG_CMD_PWM=y
 CONFIG_CMD_READ=y
 CONFIG_CMD_REMOTEPROC=y
 CONFIG_CMD_SPI=y