PCI: endpoint: Allocate enough space for fixed size BAR

PCI endpoint test function code should honor the .bar_fixed_size parameter
from underlying endpoint controller drivers or results may be unexpected.

In pci_epf_test_alloc_space(), check if BAR being used for test
register space is a fixed size BAR. If so, allocate the required fixed
size.

Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>