clk: rk3288: add PWM clock get rate

This patch add clk_get_rate for PWM device.
Signed-off-by: NKever Yang <kever.yang@rock-chips.com>
Acked-by: NSimon Glass <sjg@chromium.org>

drivers/clk/rockchip/clk_rk3288.c

@@ -695,6 +695,8 @@ static ulong rk3288_clk_get_rate(struct clk *clk)
 	case PCLK_I2C4:
 	case PCLK_I2C5:
 		return gclk_rate;
+	case PCLK_PWM:
+		return PD_BUS_PCLK_HZ;
 	default:
 		return -ENOENT;
 	}