sandbox64: add a test case for UCLASS_NVMXIP

provide a test for NVM XIP devices

The test case allows to make sure of the following:

- The NVM XIP QSPI devices are probed
- The DT entries are read correctly
- the data read from the flash by the NVMXIP block driver is correct

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index fd155ca..0257526 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1210,6 +1210,7 @@
 F:	doc/develop/driver-model/nvmxip.rst
 F:	doc/device-tree-bindings/nvmxip/nvmxip_qspi.txt
 F:	drivers/mtd/nvmxip/
+F:	test/dm/nvmxip.c
 
 NVMEM
 M:	Sean Anderson <seanga2@gmail.com>