Don't pre-reserve resources for CPU devices when they are set.

CPUs can use shared (RF_SHAREABLE) resources for the I/O port used for
entering and exiting C states. If this I/O port is included in an ACPI
system resource device, then this happens to still work, but if the port
wasn't part of a system resource device, only the first CPU could allocate
the I/O port and use C states since resource_list_reserve() was always
allocating the resource from nexus0 without RF_SHAREABLE. By avoiding
the reservation, the flags from the bus_alloc_resource() in the CPU driver
(which include RF_SHAREABLE) are honored.

PR: 236513
Reported by: stockhausen@collogia.de
Sleuthing by: avg
Reviewed by: avg
MFC after: 2 weeks