MAINTAINERS: Add Tom Rix as fpga reviewer

I take care of fpga kernel and userspace for Red Hat and would
like help out more with the mainline kernel.

Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>