[ARM] pxa/raumfeld: fix button name

This fixes a warning when booting 2.6.34-rc2:

[   26.619814] ------------[ cut here ]------------
[   26.624604] WARNING: at fs/proc/generic.c:316 __xlate_proc_name+0xac/0xc0()
[   26.631555] name 'on/off button'
[   26.634753] Modules linked in:

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>