MAINTAINERS: Add missing write-after accounts

A small number of accounts associated with maintainer roles were missing an
entry in the write-after section of the MAINTAINERS file with the
associated sourceware account.

ChangeLog:

	* MAINTAINERS (Write After): Add details for Robert Dubner,
	Ben Kosnik, James Lowden, Yunhai Shang and Andrew Waterman.
	(CPU Ports, nds32): Remove Shiva Chen - stepped down.