MAINTAINERS: remove "PKUNITY SOC DRIVERS" entry

There no PkUnity drivers left, so remove the MAINTAINERS entry.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Guenter Roeck <linux@roeck-us.net>