arch: nios2: Enable the common clk subsystem on Nios2

This patch adds support for common clock framework on Nios2. Clock
framework is commonly used in many drivers, and this patch makes it
available for the entire architecture, not just on a per-driver basis.

Signed-off-by: Beniamin Bia <[e-mail]>
Signed-off-by: Dragos Bogdan <[e-mail]>
Signed-off-by: Ley Foon Tan <[e-mail]>

arch/nios2/Kconfig

···
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
 	select ARCH_HAS_UNCACHED_SEGMENT
 	select ARCH_NO_SWAP
+	select COMMON_CLK
 	select TIMER_OF
 	select GENERIC_ATOMIC64
 	select GENERIC_CLOCKEVENTS

arch/nios2/platform/platform.c

···
 #include <linux/slab.h>
 #include <linux/sys_soc.h>
 #include <linux/io.h>
+#include <linux/clk-provider.h>
+
+static const struct of_device_id clk_match[] __initconst = {
+	{ .compatible = "fixed-clock", .data = of_fixed_clk_setup, },
+	{}
+};
 
 static int __init nios2_soc_device_init(void)
 {
···
 			kfree(soc_dev_attr);
 		}
 	}
+
+	of_clk_init(clk_match);
 
 	return 0;
 }