ACPI: EC: PM: Consolidate some code depending on PM_SLEEP

Move some routines, including acpi_ec_dispatch_gpe(), that are only
used if CONFIG_PM_SLEEP is set to the #ifdef block containing the EC
suspend and resume callbacks, to make the "full EC PM picture" easier
to follow.

While at it, move the header of acpi_ec_dispatch_gpe() in the
header file to a CONFIG_PM_SLEEP #ifdef block.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Kai-Heng Feng <kai.heng.feng@canonical.com>