test: Add test for new command pci_mps

Adds a test for the new pci_mps command to ensure that it can set the
Maximum Payload Size (MPS) of all devices to 256 bytes in the sandbox
environment. Enables the pci_mps command in the sandbox environment so
that this test can be run.

Signed-off-by: Stephen Carlson <stcarlso@linux.microsoft.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 91d40ea..d2e245e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1249,6 +1249,12 @@
 S:	Maintained
 F:	drivers/pci/pci_mpc85xx.c
 
+PCI MPS
+M:	Stephen Carlson <stcarlso@linux.microsoft.com>
+S:	Maintained
+F:	cmd/pci_mps.c
+F:	test/cmd/pci_mps.c
+
 POWER
 M:	Jaehoon Chung <jh80.chung@samsung.com>
 S:	Maintained