parisc: Make 8250_gsc driver dependend on CONFIG_PARISC

Although the name of the driver 8250_gsc.c suggests that it handles
only serial ports on the GSC bus, it does handle serial ports listed
in the parisc machine inventory as well, e.g. the serial ports in a
C8000 PCI-only workstation.

Change the dependency to CONFIG_PARISC, so that the driver gets included
in the kernel even if CONFIG_GSC isn't set.

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>