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: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>