MAINTAINERS: Drop Frank Rowand from DT maintainership

Frank retired a while back and surprisingly has found better things to
do than DT maintainership. Thank you Frank for all of your help with DT
for many years.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

MAINTAINERS

···
 
 OPEN FIRMWARE AND FLATTENED DEVICE TREE
 M:	Rob Herring <[e-mail]>
-M:	Frank Rowand <[e-mail]>
 L:	[e-mail]
 S:	Maintained
 W:	http://www.devicetree.org/