MAINTAINERS: adjust entry after renaming parisc serial driver

Commit 9e4e2ce1 ("parisc/serial: Rename 8250_gsc.c to 8250_parisc.c")
renames the parisc serial driver file, but does not adjust the entry in
MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference.

Repair this file reference in PARISC ARCHITECTURE.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Fixes: 9e4e2ce1 ("parisc/serial: Rename 8250_gsc.c to 8250_parisc.c")
Signed-off-by: Helge Deller <deller@gmx.de>