misc: pci_endpoint_test: Add support to get DMA option from userspace

'pcitest' utility now uses '-d' option to allow the user to test DMA.
Add support to parse option to use DMA from userspace application.

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