pci-epf-test/pci_endpoint_test: Cleanup PCI_ENDPOINT_TEST memspace

Cleanup PCI_ENDPOINT_TEST memspace (by moving the interrupt number away
from command section).

Add IRQ_TYPE register to identify the triggered ID interrupt required
for the READ/WRITE/COPY tests and raise IRQ test commands.

Update documentation accordingly.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>