MAINTAINERS: Add arm64 selftests to the ARM64 PORT entry

Since these are tests specific to the arm64 architecture, it makes sense
for the arm64 maintainers to gatekeep the corresponding changes.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

MAINTAINERS

@@ -2609,6 +2609,7 @@ S:	Maintained
 F:	arch/arm64/
 X:	arch/arm64/boot/dts/
 F:	Documentation/arm64/
+F:	tools/testing/selftests/arm64/
 
 AS3645A LED FLASH CONTROLLER DRIVER
 M:	Sakari Ailus <sakari.ailus@iki.fi>