clocksource/drivers/armada-370-xp: Implement ARM delay timer

Implement an ARM delay timer to be used for udelay() on Armada 37x
platforms.  This allows us to skip the delay loop calibration at boot,
saving 180ms on the boot time of the kernel (which is around 10%).

It also means that udelay() will be unaffected by CPU frequency changes
when cpufreq is enabled on these platforms.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: NGregory CLEMENT <gregory.clement@free-electrons.com>

drivers/clocksource/time-armada-370-xp.c

@@ -45,6 +45,8 @@
 #include <linux/percpu.h>
 #include <linux/syscore_ops.h>
 
+#include <asm/delay.h>
+
 /*
  * Timer block registers.
  */
@@ -249,6 +251,15 @@ struct syscore_ops armada_370_xp_timer_syscore_ops = {
 	.resume		= armada_370_xp_timer_resume,
 };
 
+static unsigned long armada_370_delay_timer_read(void)
+{
+	return ~readl(timer_base + TIMER0_VAL_OFF);
+}
+
+static struct delay_timer armada_370_delay_timer = {
+	.read_current_timer = armada_370_delay_timer_read,
+};
+
 static void __init armada_370_xp_timer_common_init(struct device_node *np)
 {
 	u32 clr = 0, set = 0;
@@ -287,6 +298,9 @@ static void __init armada_370_xp_timer_common_init(struct device_node *np)
 		TIMER0_RELOAD_EN | enable_mask,
 		TIMER0_RELOAD_EN | enable_mask);
 
+	armada_370_delay_timer.freq = timer_clk;
+	register_current_timer_delay(&armada_370_delay_timer);
+
 	/*
 	 * Set scale and timer for sched_clock.
 	 */