powerpc: xes: Add maintainer

Add Peter Tyser as the maintainer of Extreme Engineering Solutions products.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

board/xes/xpedite517x/MAINTAINERS

 XPEDITE517X BOARD
-#M:	-
+M:	Peter Tyser <ptyser@xes-inc.com>
 S:	Maintained
 F:	board/xes/xpedite517x/
 F:	include/configs/xpedite517x.h

board/xes/xpedite520x/MAINTAINERS

 XPEDITE520X BOARD
-#M:	-
+M:	Peter Tyser <ptyser@xes-inc.com>
 S:	Maintained
 F:	board/xes/xpedite520x/
 F:	include/configs/xpedite520x.h

board/xes/xpedite537x/MAINTAINERS

 XPEDITE537X BOARD
-#M:	-
+M:	Peter Tyser <ptyser@xes-inc.com>
 S:	Maintained
 F:	board/xes/xpedite537x/
 F:	include/configs/xpedite537x.h

board/xes/xpedite550x/MAINTAINERS

 XPEDITE550X BOARD
-#M:	-
+M:	Peter Tyser <ptyser@xes-inc.com>
 S:	Maintained
 F:	board/xes/xpedite550x/
 F:	include/configs/xpedite550x.h