ACPI: implement acpi_os_get_timer() according the spec

ACPI Timer() opcode should return monotonically increasing clock with 100ns
granularity according the ACPI 5.0 spec.

Testing the current Timer() implementation with following ASL code (and an
additional debug print in acpi_os_sleep() to get the sleep times dumped out
to dmesg):

	// Test: 10ms
	Store(Timer, Local1)
	Sleep(10)
	Divide(Subtract(Timer, Local1), 10000,, Local1)
	Sleep(Local1)

	// Test: 200ms
	Store(Timer, Local1)
	Sleep(200)
	Divide(Subtract(Timer, Local1), 10000,, Local1)
	Sleep(Local1)

	// Test 1300ms
	Store(Timer, Local1)
	Sleep(1300)
	Divide(Subtract(Timer, Local1), 10000,, Local1)
	Sleep(Local1)

The second sleep value is calculated using Timer(). If the implementation
is good enough we should be able to get the second value pretty close to
the first.

However, the current Timer() gives pretty bad sleep times:

	[   11.488100] ACPI: acpi_os_get_timer() TBD
	[   11.492150] ACPI: Sleep(10)
	[   11.502993] ACPI: Sleep(0)
	[   11.506315] ACPI: Sleep(200)
	[   11.706237] ACPI: Sleep(0)
	[   11.709550] ACPI: Sleep(1300)
	[   13.008929] ACPI: Sleep(0)

Fix this with the help of ktime_get(). Once the fix is applied and run
against the same ASL code we get:

	[   11.486786] ACPI: Sleep(10)
	[   11.499029] ACPI: Sleep(12)
	[   11.512350] ACPI: Sleep(200)
	[   11.712282] ACPI: Sleep(200)
	[   11.912170] ACPI: Sleep(1300)
	[   13.211577] ACPI: Sleep(1300)

That is much more closer to the values we expected.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>