atmel_spi: label GPIOs better

Make the atmel_spi driver label GPIOs according to the device for which
they're acting as a chipselect.  This way the debugfs dump of gpio state is
more informative.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>