MAINTAINERS: add myself as reviewer for SquashFS

As I have followed the development of the SquashFS support in U-Boot
as part of Joao Marcos work, it makes sense to get Cc'ed on
contributions/bug reports related to the squashfs support.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

MAINTAINERS

@@ -983,6 +983,7 @@ F:	include/spmi/
 
 SQUASHFS
 M:	Joao Marcos Costa <joaomarcos.costa@bootlin.com>
+R:	Thomas Petazzoni <thomas.petazzoni@bootlin.com>
 S:	Maintained
 F:	fs/squashfs/
 F:	include/sqfs.h