ACPI suspend: Fix CONFIG_ACPI_SLEEP dependence and some compilation warnings

Initially CONFIG_PM_SLEEP was defined as
CONFIG_SUSPEND || CONFIG_HIBERNATION and some ACPI code, most
importantly the code in drivers/acpi/main.c, was written with this
assumption.  Currently, however, CONFIG_PM_SLEEP is also set when
CONFIG_XEN_SAVE_RESTORE is set.

This causes some compilation warnings to appear in
drivers/acpi/main.c if both CONFIG_SUSPEND and CONFIG_HIBERNATION
are unset and CONFIG_PM_SLEEP is set (this was impossible before).
To fix this problem, redefine CONFIG_ACPI_SLEEP do depend directly
on CONFIG_SUSPEND || CONFIG_HIBERNATION, as originally intended, and
use it instead of CONFIG_PM_SLEEP in drivers/acpi/main.c, wherever
appropriate.

Additionally, move the acpi_target_sleep_state definition from under
the #ifdef to prevent compilation from failing in some cases.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>