MAINTAINERS: UNICORE32: Change email account

Change email account for unicore32 arch and pkunity soc drivers
Since old email server mprc.pku.edu.cn was blocked in recent years

Signed-off-by: Guan Xuetao <gxt@pku.edu.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>