misc: pci_endpoint_test: Add support to provide aligned buffer addresses

Some platforms like TI's K2G have a restriction that the host side buffer
address should be aligned to either 1MB/2MB/4MB or 8MB (Ref: 11.14.4.9.1
Outbound Address Translation in K2G TRM SPRUHY8F January 2016 – Revised May
2017) addresses depending on how it is configured in the endpoint.

Add support to provide such aligned address here so that pci_endpoint_test
driver can be used to test K2G EP.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>