parisc: add uevent helper for parisc bus

udev device-driver auto detection was failing to work on the GSC bus, since
udev didn't knew wich driver to load due to a missing MODALIAS environment
variable from kernel.

This patch fixes this by adding the MODALIAS environment variable to the
uevent kernel notifications.
Since modalias_show() generated the modalias string already, I splitted this
out and created a new static function make_modalias() which is now used by
modalias_show() and the new parisc_uevent() function.

Tested on 715/64 and c3000.
Signed-off-by: NHelge Deller <deller@gmx.de>
Signed-off-by: NKyle McMartin <kyle@mcmartin.ca>