clk: add fixed rate clock driver

This commit intends to implement "fixed-clock" as in Linux.
(drivers/clk/clk-fixed-rate.c in Linux)

If you need a very simple clock to just provide fixed clock rate
like a crystal oscillator, you do not have to write a new driver.
This driver can support it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/clk/Makefile b/drivers/clk/Makefile
index 4a6a4a8..8aa81f4 100644
--- a/drivers/clk/Makefile
+++ b/drivers/clk/Makefile
@@ -5,7 +5,7 @@
 # SPDX-License-Identifier:      GPL-2.0+
 #
 
-obj-$(CONFIG_CLK) += clk-uclass.o
+obj-$(CONFIG_CLK) += clk-uclass.o clk_fixed_rate.o
 obj-$(CONFIG_ROCKCHIP_RK3036) += clk_rk3036.o
 obj-$(CONFIG_ROCKCHIP_RK3288) += clk_rk3288.o
 obj-$(CONFIG_SANDBOX) += clk_sandbox.o