clocksource: Load the ACPI PM clocksource asynchronously

The ACPI clocksource takes quite some time to initialize,
and this increases the boot time of the kernel for a
double digit percentage. This while almost all modern
systems will be using the HPET already anyway.

This patch turns the clocksource loading into an asynchronous
operation; which means it won't hold up the boot while
still becoming available normally.

To make this work well, an udelay() had to be turned into an
usleep_range() so that on UP systems, we yield the CPU to
regular boot tasks instead of spinning.

CC: John Stultz <johnstul@us.ibm.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Len Brown <lenb@kernel.org>

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>