devmem2: use a patch that actually applies

The patch provided by Colin Didier and Maxime Hadjinlian was not
applying properly. Fixes:

  http://autobuild.buildroot.org/results/c45/c45eb706fb35ffc87fede777c3d20479a90872ae/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>