Documentation/process: Update ARM contact for embargoed hardware issues

With Grant taking a prominent role in Linaro, I will take over as the
process ambassador for ARM w.r.t. embargoed hardware issues.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Grant Likely <grant.likely@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>