MAINTAINERS: DeviceTree maintainer updates

Grant stepped down as kernel DT maintainer and his linaro.org email will
be bouncing soon, so remove him now. Pawel, Ian and Kumar either said
they don't want to remain maintainers or didn't reply, so removing them
as binding maintainers.

Update the DT git tree to mine. Grant's has not been active for a while
now.

I'm actively using patchwork for binding review tracking, so add its
URL.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>