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>

MAINTAINERS

@@ -606,7 +606,7 @@ T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-nand-flash.git
 F:	drivers/mtd/nand/raw/
 
 NDS32
-M:	Macpaul Lin <macpaul@andestech.com>
+M:	Rick Chen <rick@andestech.com>
 S:	Maintained
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-nds32.git
 F:	arch/nds32/