Fix minor locking issues in the Power Mac Uninorth PCI bridge driver.

• Call resource_int_value() once during attach, rather than within the pci_(read|write)_config() code path; this avoids taking a blocking mutex to read kenv variables.

• Use a spin lock to protect non-atomic config space accesses; this matches the behavior of Darwin's AppleMacRiscPCI driver.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D13839