MAINTAINERS: Remove Macpaul and add Rick as nds32 maintainer

Macpaul have left Andestech for a while, and this mail address
macpaul@andestech.com can not received mail anymore.

It might encounter account creation problem in this switching
to gitlab about nds32 tree.

So change the nds32 maintainer as Rick Chen <rick@andestech.com>

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>