ramips: fix cpu clock of mt7621 and add dt clk devices

For a long time the mt7621 uses a fixed cpu clock which causes a problem
if the cpu frequency is not 880MHz.

This patch fixes the cpu clock calculation and adds the cpu/bus clkdev
which will be used in dts.

Signed-off-by: Weijie Gao <hackpascal@gmail.com>

target/linux/ramips/patches-4.14/101-mt7621-timer.patch

@@ -85,22 +85,3 @@
  endchoice
  
  choice
---- a/arch/mips/ralink/timer-gic.c
-+++ b/arch/mips/ralink/timer-gic.c
-@@ -12,6 +12,7 @@
- #include <linux/of.h>
- #include <linux/clk-provider.h>
- #include <linux/clocksource.h>
-+#include <asm/time.h>
- 
- #include "common.h"
- 
-@@ -19,6 +20,8 @@ void __init plat_time_init(void)
- {
- 	ralink_of_remap();
- 
-+	mips_hpt_frequency = 880000000 / 2;
-+
- 	of_clk_init(NULL);
- 	timer_probe();
- }