MAINTAINERS: Add linux-clk patchwork URL

The common clk framework has a patchwork associated with it.
Update the maintainers file to reflect this.

Cc: Michael Turquette <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

MAINTAINERS

···
 M:	Michael Turquette <[e-mail]>
 M:	Stephen Boyd <[e-mail]>
 L:	[e-mail]
+Q:	http://patchwork.kernel.org/project/linux-clk/list/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/clk/linux.git
 S:	Maintained
 F:	Documentation/devicetree/bindings/clock/